add CLI train

[Borda]

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• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

add trainable CLI script

PR review

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[Borda]

waiting for Flash v0.5 release as the development and notebook was using a feature branch Lightning-Universe/lightning-flash#608

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[codecov]

Codecov Report

Merging #1 (c78afd0) into main (9b203e0) will decrease coverage by 4%.
The diff coverage is 54%.

@@        Coverage Diff         @@
##           main    #1   +/-   ##
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- Coverage    84%   80%   -4%     
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  Files         2     3    +1     
  Lines        76    89   +13     
==================================
+ Hits         64    71    +7     
- Misses       12    18    +6