[optim][rmsprop] group tensors in foreach to maximize perf

[janeyx99]

Test plan:
CI

[pytorch-bot]

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❌ 1 Failures

As of commit 2efec6c:

NEW FAILURES - The following jobs have failed:

• parallelnative-linux-focal-py3.7-gcc7 / test (default, 1, 2, linux.2xlarge)

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[janeyx99]

@pytorchbot merge

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[pytorchmergebot]

Merge failed

Reason: 2 additional jobs have failed, first few of them are: trunk ,trunk / parallelnative-linux-focal-py3.7-gcc7 / test (default, 1, 2, linux.2xlarge)

Details for Dev Infra team

Raised by workflow job

[janeyx99]

@pytorchbot merge -f "existing flaky test from dynamo"

[pytorchmergebot]

Merge started

Your change will be merged immediately since you used the force (-f) flag, bypassing any CI checks (ETA: 1-5 minutes).

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