Add errors for Mixed Dataloader Eval

[rishab-partha]

What does this PR do?

Now, if a user passes in a list of some DataLoaders/DataSpecs and Evaluators to the Trainer class, it no longer tries to apply all metrics to the DataLoaders/DataSpecs, causing a crash. Instead, it raises a ValueError and instructs the user to wrap the DataLoaders in an Evaluator beforehand to prevent unexpected behavior.

What issue(s) does this change relate to?

CO-1763

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[mvpatel2000]

This looks great! Mostly minor cleanup comments. Can you please also add a unit test verifying this behavior?

Also, does eval() need the same logic added to init and fit?

[rishab-partha]

fixed all above

[mvpatel2000]

LGTM! Will wait for review from @dakinggg as well though before approval

[dakinggg]

lgtm, a couple nits

[rishab-partha]

fixed nits!