Pad: don't error when unused fill value is zero
Fixes pytorch/vision#5873
In the python version of `F.pad`, checking that the fill value was
left as default was done by comparing against zero:
https://github.com/pytorch/pytorch/blob/bc2c6edaf163b1a1330e37a6e34caf8c553e4755/torch/nn/functional.py#L4366
So if someone does explicitly pass in a zero-value, then this
`TORCH_CHECK` was an accidental BC-break.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76307
Approved by: https://github.com/albanD, https://github.com/jbschlosser, https://github.com/datumbox
