Conv padding: remove string values, move to TF-style values

[stevenabreu7]

As pointed out by Terry, there is no need to support string padding values such as "same" or "valid" as "valid" is equivalent to padding=0, and "same" cannot always be implemented correctly using the PyTorch-style padding that we're currently using.

Question: should we move to Tensorflow-style padding, where the padding at the start and end of each dimension may be different? I.e. instead of (pad_x, pad_y), we specify ((pad_top, pad_bottom), (pad_left, pad_right))?

[matjobst]

I don't really see the issue here about the string paddings. Would you like the string padding values left out for reduced complexity?
Regarding the four-sided padding: That is more general than the PyTorch style. So I would be in favor of supporting it.

[stevenabreu7]

Yeah, supporting string padding is not really necessary (it's a higher level of abstraction, and an IR is not the place for that), plus it adds more complexity. It also becomes a bit arbitrary what string values we support - there are many: same, valid, full, wrap, reflect, constant, mirror, nearest, etc.

I also agree with TensorFlow-style padding. We can still allow for PyTorch-style padding, and just cast that to the TensorFlow-style in the __post_init__ (similarly to how we already do it to allow integer padding for Conv2D).