feat(tf): implement change-bias command

[Copilot]

Implements TensorFlow support for the dp change-bias command with proper checkpoint handling and variable restoration. This brings the TensorFlow backend to feature parity with the PyTorch implementation.

Key Features

• Checkpoint file support: Handles individual checkpoint files (.ckpt, .meta, .data, .index) and frozen models (.pb)
• Proper variable restoration: Variables are correctly restored from checkpoints using session initialization before bias modification
• User-defined bias support: Supports -b/--bias-value option with proper validation against model type_map
• Data-based bias calculation: Leverages existing change_energy_bias_lower functionality for automatic bias computation
• Checkpoint preservation: Saves modified variables to separate checkpoint directory for continued training
• Cross-backend consistency: Identical CLI interface and functionality as PyTorch backend

Before vs After

Variable restoration:

• Before: Change energy bias of ['O', 'H'] from [0. 0.] to [calculated values] (variables never restored)
• After: Change energy bias of ['O', 'H'] from [-93.57 -187.15] to [-93.60 -187.19] (proper restoration)

Output: Creates both updated checkpoint files AND frozen model for continued training

Documentation: Comprehensive documentation covering both TensorFlow and PyTorch backends with examples and backend-specific details

The implementation includes comprehensive test coverage with real model training to validate functionality without mocks.

Fixes #4018.

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[codecov]

Codecov Report

❌ Patch coverage is 76.31579% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.45%. Comparing base (6349238) to head (5faffab).
⚠️ Report is 9 commits behind head on devel.

Files with missing lines	Patch %	Lines
deepmd/tf/entrypoints/change_bias.py	77.18%	34 Missing ⚠️
deepmd/tf/entrypoints/main.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            devel    #4927      +/-   ##
==========================================
+ Coverage   84.29%   84.45%   +0.16%     
==========================================
  Files         703      705       +2     
  Lines       68728    69829    +1101     
  Branches     3573     3574       +1     
==========================================
+ Hits        57935    58975    +1040     
- Misses       9653     9716      +63     
+ Partials     1140     1138       -2     

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[njzjz]

@copilot The documentation change-bias.md should be revised. The current documentation only writes the PyTorch backend but not TensorFlow.

[Copilot]

@copilot The documentation change-bias.md should be revised. The current documentation only writes the PyTorch backend but not TensorFlow.

Updated the change-bias documentation to include comprehensive TensorFlow backend support. The documentation now covers both TensorFlow and PyTorch backends with:

• Backend-specific examples and usage patterns
• Supported file formats for each backend (.ckpt/.pb for TF, .pt/.pth for PyTorch)
• Common CLI parameters shared between backends
• Backend-specific implementation details

The documentation follows the established multi-backend pattern used throughout the project. (2096f7a)