Only load global step when fitting

[awaelchli]

What does this PR do?

Changes the behavior of loading the global_step.

Current:
Global step gets reloaded regardless of whether you call Trainer.fit(ckpt_path=...) or Trainer.validate(ckpt_path=...), ...

Now:
Global step will only restore its value when calling Trainer.fit(ckpt_path=...). Changed test reflects this behavior.

This is a breaking change, but aligns the behavior as expected for consistency with the rest of our reloading logic.

Discovered in #15500

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