[ENH] Add and Validate n_layers, n_units, activation & dropout_rate kwargs to MLPNetwork

[aadya940]

Closes #2337 .

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I have added the following labels to this PR based on the title: [ $\color{#FEF1BE}{\textsf{enhancement}}$ ].
I have added the following labels to this PR based on the changes made: [ $\color{#379E11}{\textsf{networks}}$ ]. Feel free to change these if they do not properly represent the PR.

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[hadifawaz1999]

Thx for taking care of it.
In general:

• All parameters should be defined as private in constructor before starting anything
• the assertions and all the list checking should only be in build_network method not constructor, this will avoid causing issues on CI (check for example how fcn network is implemented)
• Also given we are parametrizing the network, the associated classifier and regressor should be parametrized as such (also docs) and use them when calling the network
• I left some other comments to check