Add hand-only inference option

[AbedSoleymani]

This PR adds support for hand-only inference in the demo pipeline.

Changes

• Add --inference_type argument to demo.py
• Enable hand-only inference path in the estimator
• Update README.md with usage instructions
• Recommend using tightly cropped hand images for better alignment

Context

This addresses the mismatch between hand-crop inference behavior and the description in the paper (Figure 7), where the model is expected to produce hand-aligned outputs for cropped hand inputs.

Resolves #120

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[zwukong]

Cool, how is hand-only inference speed? Maybe faster than body i wonder