Support for Tensorflow 2.17

[mhuen]

Adds support for newest tensorflow version. This PR includes two updates:

• Convert KerasVariable to tensor
• fix eager execution for LR scheduler

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

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Files with missing lines	Patch %	Lines
dnn_reco/model.py	0.00%	5 Missing ⚠️
dnn_reco/utils/learning_rate.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             master     #11   +/-   ##
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  Coverage          ?   4.61%           
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  Files             ?      32           
  Lines             ?    2926           
  Branches          ?     509           
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  Hits              ?     135           
  Misses            ?    2790           
  Partials          ?       1           

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[mhuen]

Note: while falling back to compat.v1 optimizers works, support for variable learning rates is lost that was previously available. The issue is that newer tensorflow optimizers utilize keras optimizers which do not return the update operation when calling the apply_gradients method.
A proper solution is to convert everything to TF2 instead of relying on tf.compat.v1 symbols. See also #7 . This requires and extensive rewrite though.