Fixes for lightning 2.0 upgrade
# What does this PR do ?

Fix the following issues with the previous lightning 2.0 upgrade PR:

1) Decrease `val_check_interval` to 1 while resuming from a ckpt in some CI tests where max_steps is same as that of training and lightning 2.0 errors: [fit_loop.py#L259](https://github.com/Lightning-AI/lightning/blob/2.0.6/src/lightning/pytorch/loops/fit_loop.py#L259). Lightning 2.0 calls `setup_data` right at the beginning of fit_loop.run unlike 1.9 that did it only when `self.skip` for fit was False, hence the error din't show up with 1.9
2) Remove the usage of `trainer._checkpoint_connector` as it's redundant in lightning 2.0 and use `trainer.ckpt_path` directly. Also having `trainer._checkpoint_connector = _CheckpointConnector(trainer)` overrode ckpt_path with None leading to starting training from scratch during resuming from a ckpt.
3) Remove `optimizer_idx` arg in `optimizer_step` of `MegatronHalfPrecisionPlugin` class as the parent function in lightning does not have it anymore.

**Collection**: [Note which collection this PR will affect]

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