[ENH] Allow multiple instances from multiple mock classes in _safe_import

[fnhirwa]

The issue was spotted in sktime via sktime/sktime#8061

Updated the implementation and copied the unit tests to ensure that it works with pytorch-forecasting.
closes #1815

[codecov]

Codecov Report

Attention: Patch coverage is 23.63636% with 42 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@4cfa1b8). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...ting/utils/_dependencies/tests/test_safe_import.py	0.00%	36 Missing ⚠️
...ch_forecasting/utils/_dependencies/_safe_import.py	68.42%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1818   +/-   ##
=======================================
  Coverage        ?   86.20%           
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  Files           ?       47           
  Lines           ?     5357           
  Branches        ?        0           
=======================================
  Hits            ?     4618           
  Misses          ?      739           
  Partials        ?        0           

Flag	Coverage Δ
cpu	86.20% <23.63%> (?)
pytest	86.20% <23.63%> (?)

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[fnhirwa]

pinging @jgyasu
The implementation is similar to that in sktime, is there anything else I should add?

[jgyasu]

Hi @fnhirwa there have been some recent fixes concerning _safe_import in sktime and there are a few things that I still need to discuss. I think once we're done there then you can update this PR with the recent changes.

[fkiraly]

I suppose these have been merged?

[jgyasu]

I suppose these have been merged?

Yes, apologies! I do have some discussion to do around the tests but I am busy with my examinations right now so I think this can be merged and we can have a new PR later on if needed.

[fkiraly]

I updated to what I believe is in sync now with sktime.