Fix index out of bounds error for 1d convs

[DrJessop]

Summary:
Unfortunately, we are in a state where conv2d can run either 1d or 2d convs, and the padding/stride/dilation logic is pretty bad.

Essentially, if we have a conv1d, we can produce a conv2d where stride becomes [1, value_provided] in the fusion_pass (see get_conv_args). That is why in ops_registrations, we have been indexing at 1. However, if we somehow construct a conv2d_nchw/nchw without going through that fusion_pass, this canonicalization is not done.

Ideally, we have conv1d implementations where these arguments are scalars, not lists. But, this is a larger effort, since even the C++ kernels follow this convoluted logic.

Temporary fix here is to index at -1, and we'll have to come back to cleaning up conv kernels.

Differential Revision: D87943382

[pytorch-bot]

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