Bugfix: accumulation and suggestion for learning rate finder

[SkafteNicki]

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What does this PR do?

Fixes #1767, by implementing a skip_begin parameter (default 10) for the lr_finder.suggestion method

Fixes #1726, by only logging learning rate and loss when optimizer is called (as it should be). This also removes num_accumulation_steps since this is a duplicate of the accumulate_grad_batches trainer arg.

Added tests for both.

PR review

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[williamFalcon]

@SkafteNicki want to sneak this last minute into 0.7.6? we're releasing the rc2 in the next hour

[williamFalcon]

need to get these tests to pass

[Borda]

need to get these tests to pass

this is fixed In master now, just rebase it...