Add GPU support for time series models in Co-STEER (fixes #1256)

[utsab345]

GPU Support for Time Series Models

Resolves #1256

Problem

Time series models (LSTM) in Co-STEER framework were CPU-only, causing significant resource waste.

Solution

Implement automatic GPU detection and acceleration while maintaining full CPU compatibility.

Changes

• rdagent/utils/gpu_utils.py - GPU detection & utilities
• rdagent/general_model/general_model.py - Enhanced LSTM models
• rdagent/data_science/loop.py - GPU-optimized training
• rdagent/core/evolving_framework.py - Co-STEER integration
• test/utils/test_gpu_support.py - Comprehensive testing

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📚 Documentation preview 📚: https://RDAgent--1264.org.readthedocs.build/en/1264/