docs: clarify PyTorch compression support

[njzjz-bot]

Summary

• explain that TensorFlow and PyTorch model files are not interchangeable
• identify why plain dp compress parses a PyTorch model as a GraphDef
• document that DeePMD-kit 3.0.0b3 has no PyTorch compression command
• recommend uncompressed PyTorch deployment or a supported backend/version
• make the training-backend schema description version-aware

Tests

• python -m py_compile dpgen2/op/run_dp_train.py
• isort --check-only dpgen2/op/run_dp_train.py
• git diff --check

Closes #274

Coding agent: Codex
Codex version: codex-cli 0.149.0
Model: gpt-5.6-sol
Reasoning effort: xhigh

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📥 Commits

Reviewing files that changed from the base of the PR and between 6b01f29 and b8e1c39.

📒 Files selected for processing (2)

• docs/input.md
• dpgen2/op/run_dp_train.py

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.43%. Comparing base (6b01f29) to head (b8e1c39).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #401   +/-   ##
=======================================
  Coverage   84.43%   84.43%           
=======================================
  Files         104      104           
  Lines        6110     6110           
=======================================
  Hits         5159     5159           
  Misses        951      951           

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