Add size checks to `torch.stack` (#32931)
Summary:
Checks the size of each tensor passed to `torch.stack` before calling `cat` to address https://github.com/pytorch/pytorch/issues/29510. This is done in the `get_stack_input` function as that is a common path. The function now compares the size of each tensor in the TensorList to the size of the first tensor and throws an exception when the sizes are not equal.
To compare:
```
x = torch.zeros([1, 2])
y = torch.zeros([1, 3])
torch.stack([x, y]) # Errors due to size differences
```
Current error:
```
RuntimeError: invalid argument 0: Sizes of tensors must match
except in dimension 0. Got 2 and 3 in dimension 2 at (path)\aten\src\TH/generic/THTensor.cpp:612
```
New error:
```
RuntimeError: stack expects each tensor to be equal size, but
got [1, 2] at entry 0 and [1, 3] at entry 1
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32931
Differential Revision: D19700110
Pulled By: ezyang
fbshipit-source-id: 7e18bb00fa2c137e418e340d719b6b76170b83e3
