Bugfix missing variables and argument changes to MegatronPretrainingRandomSampler

[jstjohn]

What does this PR do ?

Argument changes and non-existent variables are referenced inside of the MegatronPretrainingRandomSampler are addressed.

Collection: lightning

Changelog

• Remove the dataset input that was erroneously being passed to MegatronPretrainingRandomSampler init.
• Since self.data_parallel_size doesn't actually exist inside of MegatronPretrainingRandomSampler, infer it from other variables that do exist.

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