[feature] Add a OffloadConfig object to specify offloading params to disk.

[anj-s]

What does this PR do?

Added a new class that will encompass (eventually) all parameters required for specifying offloading for FSDP. Currently we add to a list of parameters which is ever growing. This is an attempt to simplify the API and account for more knobs for given features.

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PR review

Anyone in the community is free to review the PR once the tests have passed.
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[min-xu-ai]

LGTM assuming no logic changes.

[anj-s]

LGTM assuming no logic changes.

None! Just addressing the two comments from the previous PR: removing kwargs and adding a config object + using tempfile utilities.