[ENH] Added BaseDeepForecaster in forecasting/deep_learning

[lucifer4073]

Reference Issues/PRs

Fixes #2904.

What does this implement/fix? Explain your changes.

To lay the groundwork for future deep learning forecasters in aeon, BaseDeepForecaster class has been developed. This class abstracts common deep learning workflows for forecasting, such as input conversion, sequence generation, model compilation, training, and prediction.

Does your contribution introduce a new dependency? If yes, which one?

No.

Any other comments?

This modular base class will serve as the foundation for integrating advanced deep learning models like Informer, TCN, and DeepAR into the aeon toolkit.

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I have added the following labels to this PR based on the title: [ enhancement ].
I have added the following labels to this PR based on the changes made: [ forecasting ]. Feel free to change these if they do not properly represent the PR.

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