Set parallelism_config in constructor due to Trainer reset of State

[winglian]

What does this PR do?

This is a fix for how Trainer by default resets the PartialState shared state, which means we can't read the ParallelismConfig from PartialState anymore. Since we can infer the ParallelismConfig from the env vars like we do all the other ACCELERATE_* options (e.g. FSDP, Deepspeed,etc), we can repopulate this inside the constructor.

Addtionally, there is a fix to the indent level of setting the fsdp_plugin, as if it the conditional it currently is in would bypass setting self.fsdp_plugin if the env var was set to allow standalone CP, which is independent of FSDP.

We'll need to add some fixes down the line in Trainer, but this will get us to a working release.

Fixes # (issue)

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