[Feature] Support group specification

[lucifer1004]

Description

I add an optional keyword argument group for all communication-related methods.

Motivation and Context

Close #615.

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• Bug fix (non-breaking change which fixes an issue)
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[vmoens]

@lucifer1004 I put the dist.smth type annotation in quotes because some pytorch installations don't have distributed package available (so loading tensordict will fail).
I also changed the signatures of the methods in a slightly bc-breaking way, in the sense that most args are now keyword args -- once we get many different args, we don't want to impose a clunky order, also because IMO group is more important than other args and should not be placed at the end. Happy to reconsider this!
LMK what you think of these changes!

[lucifer1004]

@vmoens These changes look good to me.