[PT-D] Use process group of the partial tensor so sub pg comm will be enabled during reshard

[fduwjj]

Stack from ghstack (oldest at bottom):

• -> [PT-D] Use process group of the partial tensor so sub pg comm will be enabled during reshard #79357

During the debugging for TP enablement for Transformer model, looks like Partial tensor does not use its own pg during resharding while using the default pg instead. Switch to use its own pg as the fix.

Differential Revision: D37093468

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[fduwjj]

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[fduwjj]

@pytorchbot merge

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[github-actions]

Hey @fduwjj.
You've committed this PR, but it does not have both a 'release notes: ...' and 'topics: ...' label. Please add one of each to the PR. The 'release notes: ...' label should represent the part of PyTorch that this PR changes (fx, autograd, distributed, etc) and the 'topics: ...' label should represent the kind of PR it is (not user facing, new feature, bug fix, perf improvement, etc). The list of valid labels can be found here for the 'release notes: ...' and here for the 'topics: ...'.
For changes that are 'topic: not user facing' there is no need for a release notes label.