Refactor access to trainer attributes in LightningModule

[awaelchli]

What does this PR do?

Implements read-only passthrough for the attributes that are exclusively handled by the Trainer. This avoids accidental divergence of state. Additionally, just to be safe we make the trainer a week-ref.

Refactor is also partially necessary for #5616

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[codecov]

Codecov Report

Merging #5730 (6ef068a) into release/1.2-dev (069ae27) will decrease coverage by 2%.
The diff coverage is 100%.

@@               Coverage Diff                @@
##           release/1.2-dev   #5730    +/-   ##
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- Coverage               89%     87%    -2%     
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  Files                  173     173            
  Lines                12495   12758   +263     
================================================
- Hits                 11171   11089    -82     
- Misses                1324    1669   +345     

[tchaton]

LGTM !