feat(pt): implement shared_dict functionality for linear models

[Copilot]

This PR implements shared_dict parameter support for LinearEnergyModel to enable parameter sharing between submodels, following the same pattern used in multi-task models.

Problem

Users requested the ability to share parameters between submodels in linear models, similar to how multi-task models work. The original linear model implementation only supported independent submodels without parameter sharing capabilities.

Solution

Added support for the shared_dict parameter to LinearEnergyModel by:

1. Parameter preprocessing: Integrated preprocess_shared_params() to convert shared references to actual parameter dictionaries
2. Parameter sharing logic: Implemented _share_params() method adapted from ModelWrapper.share_params() to enable actual parameter sharing between submodels
3. Backward compatibility: All existing linear model usage patterns continue to work unchanged

Usage Example

config = {
    "type": "linear_ener",
    "shared_dict": {
        "type_map_all": ["O", "H"],
        "dpa1_descriptor_1": {
            "type": "dpa1",
            "rcut": 6.00,
            "rcut_smth": 0.50,
            "sel": 138,
            "neuron": [25, 50, 100],
            "axis_neuron": 16,
            "seed": 1,
        },
    },
    "models": [
        {
            "type_map": "type_map_all",
            "descriptor": "dpa1_descriptor_1",
            "fitting_net": {
                "neuron": [240, 240, 240],
                "resnet_dt": True,
                "seed": 1,
            },
        },
        {
            "type_map": "type_map_all", 
            "descriptor": "dpa1_descriptor_1",
            "fitting_net": {
                "type": "dipole",
                "neuron": [100, 100, 100],
                "resnet_dt": True,
                "seed": 1,
            },
        },
    ],
    "weights": "mean",
}

model = LinearEnergyModel(**config)

The implementation reuses existing multi-task infrastructure to minimize code changes while providing the requested functionality.

Fixes #4412.

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[njzjz]

@copilot continue your work.

[Copilot]

@copilot continue your work.

Continuing work on the shared_dict implementation for linear models. I've completed the core implementation and testing:

✅ LinearEnergyModel now supports shared_dict parameter
✅ Parameter sharing logic integrated using existing multi-task patterns
✅ Tests added and passing for both shared_dict and backward compatibility
✅ Issue #4412 example configuration confirmed working
✅ Existing tests still pass (no regressions)
✅ Code properly linted and formatted

The implementation allows parameter sharing between submodels in linear models following the same pattern as multi-task models, with minimal changes to the codebase.