Make MvStudentT distribution v4 compatible (#4731)

* Make MvStudentT distribution v4 compatible

* Enable pymc3/tests/test_distributions.py::TestBugfixes::test_issue_3051

* refactor tests

* simplify rng_fn method and refactor tests

* Refactor rng method to compute sqrt over chi2_samples, one step earlier

* Modify rng method to use aesara multivariate_normal.rng_fn

* Add suggestions

Co-authored-by: Ricardo <ricardo.vieira1994@gmail.com>

* Add a check against negative support of degrees of freedom

Co-authored-by: Ricardo <ricardo.vieira1994@gmail.com>

Co-authored-by: Ricardo <ricardo.vieira1994@gmail.com>

Author: Sayam753

pymc3/distributions/multivariate.py

@@ -27,6 +27,7 @@
 from aesara.tensor import gammaln
 from aesara.tensor.nlinalg import det, eigh, matrix_inverse, trace
 from aesara.tensor.random.basic import MultinomialRV, dirichlet, multivariate_normal
+from aesara.tensor.random.op import RandomVariable, default_shape_from_params
 from aesara.tensor.random.utils import broadcast_params
 from aesara.tensor.slinalg import (
     Cholesky,
@@ -41,7 +42,7 @@
 
 from pymc3.aesaraf import floatX, intX
 from pymc3.distributions import transforms
-from pymc3.distributions.continuous import ChiSquared, Normal
+from pymc3.distributions.continuous import ChiSquared, Normal, assert_negative_support
 from pymc3.distributions.dist_math import bound, factln, logpow, multigammaln
 from pymc3.distributions.distribution import Continuous, Discrete
 from pymc3.math import kron_diag, kron_dot, kron_solve_lower, kronecker
@@ -248,6 +249,48 @@ def _distr_parameters_for_repr(self):
         return ["mu", "cov"]
 
 
+class MvStudentTRV(RandomVariable):
+    name = "multivariate_studentt"
+    ndim_supp = 1
+    ndims_params = [0, 1, 2]
+    dtype = "floatX"
+    _print_name = ("MvStudentT", "\\operatorname{MvStudentT}")
+
+    def __call__(self, nu, mu=None, cov=None, size=None, **kwargs):
+
+        dtype = aesara.config.floatX if self.dtype == "floatX" else self.dtype
+
+        if mu is None:
+            mu = np.array([0.0], dtype=dtype)
+        if cov is None:
+            cov = np.array([[1.0]], dtype=dtype)
+        return super().__call__(nu, mu, cov, size=size, **kwargs)
+
+    def _shape_from_params(self, dist_params, rep_param_idx=1, param_shapes=None):
+        return default_shape_from_params(self.ndim_supp, dist_params, rep_param_idx, param_shapes)
+
+    @classmethod
+    def rng_fn(cls, rng, nu, mu, cov, size):
+
+        # Don't reassign broadcasted cov, since MvNormal expects two dimensional cov only.
+        mu, _ = broadcast_params([mu, cov], cls.ndims_params[1:])
+
+        chi2_samples = np.sqrt(rng.chisquare(nu, size=size) / nu)
+        # Add distribution shape to chi2 samples
+        chi2_samples = chi2_samples.reshape(chi2_samples.shape + (1,) * len(mu.shape))
+
+        mv_samples = multivariate_normal.rng_fn(rng=rng, mean=np.zeros_like(mu), cov=cov, size=size)
+
+        size = tuple(size or ())
+        if size:
+            mu = np.broadcast_to(mu, size + mu.shape)
+
+        return (mv_samples / chi2_samples) + mu
+
+
+mv_studentt = MvStudentTRV()
+
+
 class MvStudentT(Continuous):
     r"""
     Multivariate Student-T log-likelihood.
@@ -273,8 +316,8 @@ class MvStudentT(Continuous):
 
     Parameters
     ----------
-    nu: int
-        Degrees of freedom.
+    nu: float
+        Degrees of freedom, should be a positive scalar.
     Sigma: matrix
         Covariance matrix. Use `cov` in new code.
     mu: array
@@ -288,55 +331,21 @@ class MvStudentT(Continuous):
     lower: bool, default=True
         Whether the cholesky fatcor is given as a lower triangular matrix.
     """
+    rv_op = mv_studentt
 
-    def __init__(
-        self, nu, Sigma=None, mu=None, cov=None, tau=None, chol=None, lower=True, *args, **kwargs
-    ):
+    @classmethod
+    def dist(cls, nu, Sigma=None, mu=None, cov=None, tau=None, chol=None, lower=True, **kwargs):
         if Sigma is not None:
             if cov is not None:
                 raise ValueError("Specify only one of cov and Sigma")
             cov = Sigma
-        super().__init__(mu=mu, cov=cov, tau=tau, chol=chol, lower=lower, *args, **kwargs)
-        self.nu = nu = at.as_tensor_variable(nu)
-        self.mean = self.median = self.mode = self.mu = self.mu
-
-    def random(self, point=None, size=None):
-        """
-        Draw random values from Multivariate Student's T distribution.
-
-        Parameters
-        ----------
-        point: dict, optional
-            Dict of variable values on which random values are to be
-            conditioned (uses default point if not specified).
-        size: int, optional
-            Desired size of random sample (returns one sample if not
-            specified).
-
-        Returns
-        -------
-        array
-        """
-        # with _DrawValuesContext():
-        #     nu, mu = draw_values([self.nu, self.mu], point=point, size=size)
-        #     if self._cov_type == "cov":
-        #         (cov,) = draw_values([self.cov], point=point, size=size)
-        #         dist = MvNormal.dist(mu=np.zeros_like(mu), cov=cov, shape=self.shape)
-        #     elif self._cov_type == "tau":
-        #         (tau,) = draw_values([self.tau], point=point, size=size)
-        #         dist = MvNormal.dist(mu=np.zeros_like(mu), tau=tau, shape=self.shape)
-        #     else:
-        #         (chol,) = draw_values([self.chol_cov], point=point, size=size)
-        #         dist = MvNormal.dist(mu=np.zeros_like(mu), chol=chol, shape=self.shape)
-        #
-        #     samples = dist.random(point, size)
-        #
-        # chi2_samples = np.random.chisquare(nu, size)
-        # # Add distribution shape to chi2 samples
-        # chi2_samples = chi2_samples.reshape(chi2_samples.shape + (1,) * len(self.shape))
-        # return (samples / np.sqrt(chi2_samples / nu)) + mu
+        nu = at.as_tensor_variable(floatX(nu))
+        mu = at.as_tensor_variable(floatX(mu))
+        cov = quaddist_matrix(cov, chol, tau, lower)
+        assert_negative_support(nu, "nu", "MvStudentT")
+        return super().dist([nu, mu, cov], **kwargs)
 
-    def logp(value, nu, cov):
+    def logp(value, nu, mu, cov):
         """
         Calculate log-probability of Multivariate Student's T distribution
         at specified value.
@@ -350,15 +359,15 @@ def logp(value, nu, cov):
         -------
         TensorVariable
         """
-        quaddist, logdet, ok = quaddist_parse(value, nu, cov)
+        quaddist, logdet, ok = quaddist_parse(value, mu, cov)
         k = floatX(value.shape[-1])
 
-        norm = gammaln((nu + k) / 2.0) - gammaln(nu / 2.0) - 0.5 * k * floatX(np.log(nu * np.pi))
+        norm = gammaln((nu + k) / 2.0) - gammaln(nu / 2.0) - 0.5 * k * at.log(nu * np.pi)
         inner = -(nu + k) / 2.0 * at.log1p(quaddist / nu)
         return bound(norm + inner - logdet, ok)
 
     def _distr_parameters_for_repr(self):
-        return ["mu", "nu", "cov"]
+        return ["nu", "mu", "cov"]
 
 
 class Dirichlet(Continuous):

pymc3/tests/test_distributions.py

@@ -2017,7 +2017,6 @@ def test_kroneckernormal(self, n, m, sigma):
         )
 
     @pytest.mark.parametrize("n", [1, 2])
-    @pytest.mark.xfail(reason="Distribution not refactored yet")
     def test_mvt(self, n):
         self.check_logp(
             MvStudentT,
@@ -2030,6 +2029,7 @@ def test_mvt(self, n):
             RealMatrix(2, n),
             {"nu": Rplus, "Sigma": PdMatrix(n), "mu": Vector(R, n)},
             mvt_logpdf,
+            extra_args={"size": 2},
         )
 
     @pytest.mark.parametrize("n", [2, 3, 4])
@@ -2936,13 +2936,11 @@ def test_car_logp(size):
 
 
 class TestBugfixes:
-    @pytest.mark.parametrize(
-        "dist_cls,kwargs", [(MvNormal, dict(mu=0)), (MvStudentT, dict(mu=0, nu=2))]
-    )
+    @pytest.mark.parametrize("dist_cls,kwargs", [(MvNormal, dict()), (MvStudentT, dict(nu=2))])
     @pytest.mark.parametrize("dims", [1, 2, 4])
-    @pytest.mark.xfail(reason="Distribution not refactored yet")
     def test_issue_3051(self, dims, dist_cls, kwargs):
-        d = dist_cls.dist(**kwargs, cov=np.eye(dims), size=(dims,))
+        mu = np.repeat(0, dims)
+        d = dist_cls.dist(mu=mu, cov=np.eye(dims), **kwargs, size=(20))
 
         X = np.random.normal(size=(20, dims))
         actual_t = logpt(d, X)

pymc3/tests/test_distributions_random.py

@@ -889,7 +889,7 @@ class TestPoisson(BaseTestDistribution):
     tests_to_run = ["check_pymc_params_match_rv_op"]
 
 
-class TestMvNormal(BaseTestDistribution):
+class TestMvNormalCov(BaseTestDistribution):
     pymc_dist = pm.MvNormal
     pymc_dist_params = {
         "mu": np.array([1.0, 2.0]),
@@ -939,6 +939,70 @@ class TestMvNormalTau(BaseTestDistribution):
     tests_to_run = ["check_pymc_params_match_rv_op"]
 
 
+class TestMvStudentTCov(BaseTestDistribution):
+    def mvstudentt_rng_fn(self, size, nu, mu, cov, rng):
+        chi2_samples = rng.chisquare(nu, size=size)
+        mv_samples = rng.multivariate_normal(np.zeros_like(mu), cov, size=size)
+        return (mv_samples / np.sqrt(chi2_samples[:, None] / nu)) + mu
+
+    pymc_dist = pm.MvStudentT
+    pymc_dist_params = {
+        "nu": 5,
+        "mu": np.array([1.0, 2.0]),
+        "cov": np.array([[2.0, 0.0], [0.0, 3.5]]),
+    }
+    expected_rv_op_params = {
+        "nu": 5,
+        "mu": np.array([1.0, 2.0]),
+        "cov": np.array([[2.0, 0.0], [0.0, 3.5]]),
+    }
+    sizes_to_check = [None, (1), (2, 3)]
+    sizes_expected = [(2,), (1, 2), (2, 3, 2)]
+    reference_dist_params = {
+        "nu": 5,
+        "mu": np.array([1.0, 2.0]),
+        "cov": np.array([[2.0, 0.0], [0.0, 3.5]]),
+    }
+    reference_dist = lambda self: functools.partial(
+        self.mvstudentt_rng_fn, rng=self.get_random_state()
+    )
+    tests_to_run = [
+        "check_pymc_params_match_rv_op",
+        "check_pymc_draws_match_reference",
+        "check_rv_size",
+    ]
+
+
+class TestMvStudentTChol(BaseTestDistribution):
+    pymc_dist = pm.MvStudentT
+    pymc_dist_params = {
+        "nu": 5,
+        "mu": np.array([1.0, 2.0]),
+        "chol": np.array([[2.0, 0.0], [0.0, 3.5]]),
+    }
+    expected_rv_op_params = {
+        "nu": 5,
+        "mu": np.array([1.0, 2.0]),
+        "cov": quaddist_matrix(chol=pymc_dist_params["chol"]).eval(),
+    }
+    tests_to_run = ["check_pymc_params_match_rv_op"]
+
+
+class TestMvStudentTTau(BaseTestDistribution):
+    pymc_dist = pm.MvStudentT
+    pymc_dist_params = {
+        "nu": 5,
+        "mu": np.array([1.0, 2.0]),
+        "tau": np.array([[2.0, 0.0], [0.0, 3.5]]),
+    }
+    expected_rv_op_params = {
+        "nu": 5,
+        "mu": np.array([1.0, 2.0]),
+        "cov": quaddist_matrix(tau=pymc_dist_params["tau"]).eval(),
+    }
+    tests_to_run = ["check_pymc_params_match_rv_op"]
+
+
 class TestDirichlet(BaseTestDistribution):
     pymc_dist = pm.Dirichlet
     pymc_dist_params = {"a": np.array([1.0, 2.0])}
@@ -1471,22 +1535,6 @@ def ref_rand_evd(size, mu, evds, sigma):
                 model_args=evd_args,
             )
 
-    @pytest.mark.xfail(reason="This distribution has not been refactored for v4")
-    def test_mv_t(self):
-        def ref_rand(size, nu, Sigma, mu):
-            normal = st.multivariate_normal.rvs(cov=Sigma, size=size)
-            chi2 = st.chi2.rvs(df=nu, size=size)[..., None]
-            return mu + (normal / np.sqrt(chi2 / nu))
-
-        for n in [2, 3]:
-            pymc3_random(
-                pm.MvStudentT,
-                {"nu": Domain([5, 10, 25, 50]), "Sigma": PdMatrix(n), "mu": Vector(R, n)},
-                size=100,
-                valuedomain=Vector(R, n),
-                ref_rand=ref_rand,
-            )
-
     @pytest.mark.xfail(reason="This distribution has not been refactored for v4")
     def test_dirichlet_multinomial(self):
         def ref_rand(size, a, n):