Remove [None, :]

[gab-chen]

Description of changes:
Minor code cleaning.
The [None, :] is unnecessary, because without [None, :] PyTorch broadcasting will also prepend 1 to the dimensions of torch.arange()'s output.

I tested the change by removing the [None, :] and running the masked_softmax()code below in 11.3.2.1.
The output are the same as when we have [None, :], in terms of the numbers of masked and unmasked elements.
I pasted the output below. You can compare it with the output on the website.

masked_softmax(torch.rand(2, 2, 4), torch.tensor([2, 3]))
tensor([[[0.5670, 0.4330, 0.0000, 0.0000],
         [0.5983, 0.4017, 0.0000, 0.0000]],

        [[0.4297, 0.3518, 0.2185, 0.0000],
         [0.3578, 0.3347, 0.3075, 0.0000]]])

masked_softmax(torch.rand(2, 2, 4), torch.tensor([[1, 3], [2, 4]]))
tensor([[[1.0000, 0.0000, 0.0000, 0.0000],
         [0.4129, 0.3338, 0.2533, 0.0000]],

        [[0.4291, 0.5709, 0.0000, 0.0000],
         [0.2964, 0.2290, 0.1903, 0.2844]]])

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[d2l-bot]

Job d2l-en/PR-2418/1 is complete.
Check the results at http://preview.d2l.ai/d2l-en/PR-2418/

[d2l-bot]

Job d2l-en/PR-2418/2 is complete.
Check the results at http://preview.d2l.ai/d2l-en/PR-2418/