Feature: Introduce ML_Base class and add ML_EXX support

[sunliang98]

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• Have you added adequate unit tests and/or case tests for your pull request?
• Have you noticed possible changes of behavior below or in the linked issue?
• Have you explained the changes of codes in core modules of ESolver, HSolver, ElecState, Hamilt, Operator or Psi? (ignore if not applicable)

Linked Issue

Fix #...

Unit Tests and/or Case Tests for my changes

• A unit test is added for each new feature or bug fix.

What's changed?

This PR introduces the following updates:

1. Defines the ML_Base class containing common methods for machine-learning functionals, making the original KEDF_ML class inherit from it.
2. Implements the ML_EXX class, a subclass of ML_Base, for computing exact exchange energy.
3. Integrates ML_EXX into the DFT workflow, currently supporting KSDFT with PW and LCAO basis sets, as well as OFDFT calls.
4. Adds the input parameter ml_exx to control the activation of the ML_EXX feature.

Note: This feature is currently under development. Future improvements include:

• Decoupling neural network descriptors from ML_KEDF parameters.
• Extending support from SPIN 1 to SPIN 2 cases.
• Enabling the generation of training data for exchange energy.

Any changes of core modules? (ignore if not applicable)

• Example: I have added a new virtual function in the esolver base class in order to ...