fix: fix overlap param gather

[ashors1]

What does this PR do ?

When using overlap param gather, Mcore does a single param all-gather after each backward pass (before the first forward step). This is problematic when the first forward pass following a backward pass is using the reference model parameters, because we end up doing an all-gather on the reference parameters rather than the model being trained. The solution is to disable overlap param gather when running reference model forward and re-enable after.

Issues

Closes #552

Usage

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