Add state validation to QuantumDevice.set_states()

[yurekami]

Summary

Add optional check_legality parameter to set_states() that validates:

• Batch size consistency between input states and device
• State normalization (|amplitudes|² should sum to 1)

Problem

As noted in #305, users can set states with mismatched batch sizes or unnormalized states without any warning, leading to confusion and incorrect results.

Solution

Add opt-in validation with informative warnings:

q_device = tq.QuantumDevice(n_wires=2, bsz=1)

# Without validation (default, backwards compatible)
q_device.set_states(states)

# With validation
q_device.set_states(states, check_legality=True)
# Warning: Input states batch size (3) differs from device bsz (1)...
# Warning: Input states are not normalized. Norms: [103.0]...

The device's bsz is updated to match the input states' batch size.

Test plan

• Verify warning is issued for mismatched batch size
• Verify warning is issued for unnormalized states
• Verify no warnings when check_legality=False (default)

Fixes #305

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