Make trainer.state a read-only property

[zerogerc]

What does this PR do?

Follow-up on #2541 to make trainer state read-only.

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[codecov]

Codecov Report

Merging #3109 into master will decrease coverage by 1%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #3109    +/-   ##
=======================================
- Coverage      90%     89%    -1%     
=======================================
  Files          81      82     +1     
  Lines        7780    8320   +540     
=======================================
+ Hits         6995    7410   +415     
- Misses        785     910   +125     

[Borda]

good work, but I would rather add also setter which main checks that the values in TrainerState type :]

[justusschock]

@Borda I would prefer to keep it read-only as well. everything that is exposed will most likely be used by users in some way. And checking for valid arguments is quite hard. We can check for types, but even arguments with correct types may be wrong w.r.t. to trainer's flow.

[Borda]

self._state = exiting if exiting is not None else state_before

[zerogerc]

Thanks for fixing this =)

[justusschock]

To me this looks completely fine :D

Making things, the user isn't expected to change directly read-only is always a good idea I think :D

[SkafteNicki]

LGTM

[awaelchli]

I like to see this ❤️