Consider even block sizes in drop block 2d

[dcasbol]

In the current version, drop_block2d only seems to work with block sizes that are odd-sized i.e. 3, 5, 7, 9, ... However, this is not covered in the documentation so it took me a bit to figure out. Fixing it requires applying asymmetric padding, which max_pool2d doesn't support, so an explicit padding via F.pad is necessary in this case. Also, I reckon that it might be much simpler to comment this aspect in the documentation and include a check in the code than accepting even block sizes.

This fix only applies to the 2d version and does not fix drop_block3d.

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