Assert tensor isn't sparse in enforce_invariants. (pytorch#18338)

Summary:
There's no reason we can't check this, but I'm punting on implementing it for now.  But it currently segfaults, so this is an improvements.
Pull Request resolved: pytorch#18338

Differential Revision: D14580308

Pulled By: gchanan

fbshipit-source-id: 44d4cafeab12e1beeb3453a2d4068d221c2e9c4f

Author: gchanan

aten/src/ATen/core/Tensor.cpp

@@ -19,6 +19,9 @@ void Tensor::enforce_invariants() {
       AT_ASSERTM(
           impl_->dtype_initialized(),
           "Partially-initialized tensor not supported by at::Tensor");
+      AT_ASSERTM(
+          !impl_->is_sparse(),
+          "Sparse Tensors are supported by at::Tensor, but invariant checking isn't implemented.  Please file a bug.");
       AT_ASSERTM(
           impl_->storage_initialized(),
           "Partially-initialized tensor not supported by at::Tensor");