Pass config to constructor when reviving custom functional model

[TrAyZeN]

Currently, when loading a model instantiated from a custom Model subclass its config is not passed to it's constructor. This leads to some parameters not being restored.

Here is a snippet showing the behavior mentioned:

class CustomModel(Model):
    def __init__(self, *args, param=1, **kwargs):
        super().__init__(*args, **kwargs)
        self.param = param

    def get_config(self):
        base_config = super().get_config()
        config = {"param": self.param}
        return base_config | config

inputs = layers.Input((3,))
outputs = layers.Dense(5)(inputs)
model = CustomModel(inputs=inputs, outputs=outputs, param=3)

new_model = CustomModel.from_config(model.get_config())
print(new_model.param) # prints 1 currently i.e. default value of param

This PR proposes to fix this issue by passing config in functional_from_config to the model constructor.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.87%. Comparing base (ca88613) to head (0be55ca).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #20321      +/-   ##
==========================================
+ Coverage   78.81%   78.87%   +0.06%     
==========================================
  Files         512      513       +1     
  Lines       49056    49236     +180     
  Branches     9033     9076      +43     
==========================================
+ Hits        38664    38837     +173     
- Misses       8528     8532       +4     
- Partials     1864     1867       +3     

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keras-jax	62.38% <100.00%> (+0.11%)	⬆️
keras-numpy	57.40% <100.00%> (-0.01%)	⬇️
keras-tensorflow	63.64% <100.00%> (+0.08%)	⬆️
keras-torch	62.37% <100.00%> (+0.11%)	⬆️

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[fchollet]

Thanks for the PR -- looks good to me!

[fchollet]

You can simply this block:

• single iteration over set of keys
• use pop(key, None)

[TrAyZeN]

I have applied your suggestion, I like the idea to simplify it. However, it does not keep the previous behavior. Now instead of throwing a KeyError if layers, input_layers or output_layers is not present, it will silently set its value to None. It doesn't seem to be an issue, but I just wanted to mention it.

[fchollet]

You're right, it would lead to a strange error message instead of a clear one if the config is malformed. Let's avoid that.

[TrAyZeN]

I reverted the commit then. I am still waiting for the CLA to be handled. I'll let you know when that's done.