Add padding='same' mode to conv{1,2,3}d (#45667)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45667
First part of #3867 (Pooling operators still to do)
This adds a `padding='same'` mode to the interface of `conv{n}d`and `nn.Conv{n}d`. This should match the behaviour of `tensorflow`. I couldn't find it explicitly documented but through experimentation I found `tensorflow` returns the shape `ceil(len/stride)` and always adds any extra asymmetric padding onto the right side of the input.
Since the `native_functions.yaml` schema doesn't seem to support strings or enums, I've moved the function interface into python and it now dispatches between the numerically padded `conv{n}d` and the `_conv{n}d_same` variant. Underscores because I couldn't see any way to avoid exporting a function into the `torch` namespace.
A note on asymmetric padding. The total padding required can be odd if both the kernel-length is even and the dilation is odd. mkldnn has native support for asymmetric padding, so there is no overhead there, but for other backends I resort to padding the input tensor by 1 on the right hand side to make the remaining padding symmetrical. In these cases, I use `TORCH_WARN_ONCE` to notify the user of the performance implications.
Test Plan: Imported from OSS
Reviewed By: ejguan
Differential Revision: D27170744
Pulled By: jbschlosser
fbshipit-source-id: b3d8a0380e0787ae781f2e5d8ee365a7bfd49f22
