fix error on Python 3.7 unpacking tuple in return statements

[susumuota]

What does this PR do?

On Python 3.7.15 (current Google Colab), #527 still raise SyntexError.

  File "/usr/local/lib/python3.7/dist-packages/xformers/ops/unbind.py", line 99
    return None, *(grad.unbind(dim=ctx.dim))
                 ^
SyntaxError: invalid syntax

According to these info,

• https://bugs.python.org/issue32117
• https://stackoverflow.com/questions/47272460/why-is-starred-iterable-unpacking-in-a-return-statement-invalid-syntax-without-p

It is a matter of the "return statement", not only unpacking things. So it should be,

        return (None, *grad.unbind(dim=ctx.dim))

I confirmed this fix works on 3.7.15.

It was discussed at 0428e12#r91033157

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PR review

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[danthe3rd]

Thanks a lot! Will merge once the tests pass