Fix layerwise expt dp size 1 and contention with element wise distributed optimizer

[skyw]

What does this PR do ?

fix couple of issues:

• In rare case expt dp size 1, bucket.layerwise_param_flat_sizes can be None and cause later code to crash.
• should_disable_forward_pre_hook() in traing.py depends on deprecated "dist_" prefix of optimizer name.
• Some checks of use_distributed_optimizer in arguments.py can fail unnecessarily with layer wise.

A further clean up of making element wise distributed optimizer more clear is also planned.

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[skyw]

/ok to test 17747cb

[skyw]

/ok to test 48f891c

[copy-pr-bot]

/ok to test 48f891c

@skyw, there was an error processing your request: E2

See the following link for more information: https://docs.gha-runners.nvidia.com/cpr/e/2/

[skyw]

/ok to test 42ca188

[mchrzanowski]

can you add some unit tests to make sure dp=1 continues to work?

lgtm modulo request for tests