Fix distributed store to use add for the counter of DL shared seed

[ejguan]

In order to get the result of _shared_seed_recv_cnt properly, switch from store.get to store.add(key, 0).

See the comment from distributed team for the reason:

pytorch/torch/distributed/distributed_c10d.py

Lines 242 to 246 in 590d3e5

	# Use 'add' instead of 'get' since for some store implementations 'add'
	# doesn't work well with 'get'. Ideally the store implementations should
	# be fixed, but for backward compatiblity reasons it is risky to change
	# the store implementations. Once, we completely migrate away from these
	# legacy stores, we can use 'get' here instead.

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@ejguan has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

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[ejguan]

@pytorchbot merge -g

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[github-actions]

Hey @ejguan.
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.