[einsum] keep the promise that we contract left to right (pytorch#87199)

We promise that if path is not defined, we would go left to right. The previous code did not keep that promise as we push'd combined ops to the back of the list. For most use cases this is fine (einsum with 3 or fewer inputs), but we should do what we say.

Test plan:
Added a print statement to print the sizes of ops we're contracting to see if the order is fixed. Code run:
```
import torch
a = torch.rand(1)
b = torch.rand(2)
c = torch.rand(3)
d = torch.rand(4)
torch.einsum('a,b,c,d->abcd', a,b,c,d)
```

BEFORE--it does a+b, then c+d, then a+b+c+d, which...is right, but it's not the order specified by the user.
```
/Users/janeyx/pytorch/torch/functional.py:378: UserWarning: Contracting a: [1, 1, 1, 1]and b: [1, 2, 1, 1] (Triggered internally at /Users/janeyx/pytorch/aten/src/ATen/native/Linear.cpp:507.)
  return _VF.einsum(equation, operands)  # type: ignore[attr-defined]
/Users/janeyx/pytorch/torch/functional.py:378: UserWarning: Contracting a: [1, 1, 3, 1]and b: [1, 1, 1, 4] (Triggered internally at /Users/janeyx/pytorch/aten/src/ATen/native/Linear.cpp:507.)
  return _VF.einsum(equation, operands)  # type: ignore[attr-defined]
/Users/janeyx/pytorch/torch/functional.py:378: UserWarning: Contracting a: [1, 2, 1, 1]and b: [1, 1, 3, 4] (Triggered internally at /Users/janeyx/pytorch/aten/src/ATen/native/Linear.cpp:507.)
  return _VF.einsum(equation, operands)  # type: ignore[attr-defined]
```

WITH THIS CHANGE--it actually goes left to right: a+b, a+b+c, a+b+c+d
```
/Users/janeyx/pytorch/torch/functional.py:378: UserWarning: Contracting a: [1, 1, 1, 1]and b: [1, 2, 1, 1] (Triggered internally at /Users/janeyx/pytorch/aten/src/ATen/native/Linear.cpp:507.)
  return _VF.einsum(equation, operands)  # type: ignore[attr-defined]
/Users/janeyx/pytorch/torch/functional.py:378: UserWarning: Contracting a: [1, 2, 1, 1]and b: [1, 1, 3, 1] (Triggered internally at /Users/janeyx/pytorch/aten/src/ATen/native/Linear.cpp:507.)
  return _VF.einsum(equation, operands)  # type: ignore[attr-defined]
/Users/janeyx/pytorch/torch/functional.py:378: UserWarning: Contracting a: [1, 2, 3, 1]and b: [1, 1, 1, 4] (Triggered internally at /Users/janeyx/pytorch/aten/src/ATen/native/Linear.cpp:507.)
  return _VF.einsum(equation, operands)  # type: ignore[attr-defined]
```
Pull Request resolved: pytorch#87199
Approved by: https://github.com/soulitzer

Author: janeyx99

aten/src/ATen/native/Linear.cpp

@@ -405,7 +405,7 @@ Tensor einsum(c10::string_view equation, TensorList operands, at::OptionalIntArr
   std::vector<SymInt> label_size(TOTAL_LABELS, 1);
   std::vector<SymInt> ell_sizes(ell_num_dim, 1);
   std::vector<uint64_t> dim_counts(perm_index, 0);
-  std::vector<Tensor> ops;
+  std::deque<Tensor> ops;
   for (const auto i : irange(num_ops)) {
     auto op = operands[i];
     std::vector<int64_t> permutation(perm_index, -1);
@@ -536,7 +536,11 @@ Tensor einsum(c10::string_view equation, TensorList operands, at::OptionalIntArr
       b = b.sum(b_dims_to_sum, true);
     }
 
-    ops.emplace_back(sumproduct_pair(a, b, sum_dims, true));
+    if (path.has_value()) {
+      ops.emplace_back(sumproduct_pair(a, b, sum_dims, true));
+    } else {
+      ops.emplace_front(sumproduct_pair(a, b, sum_dims, true));
+    }
   }
 
   // Sum out contraction dims