Fix NoiseDevice buffer device placement

[yurekami]

Summary

Added .to(self.device) when initializing the density tensor in NoiseDevice to match the behavior of QuantumDevice.

Problem

NoiseDevice buffers remained on CPU even when device='cuda' was specified:

# In NoiseDevice.__init__ (before fix):
_density = torch.zeros(2 ** (2 * self.n_wires), dtype=C_DTYPE)
_density[0] = 1 + 0j
_density = torch.reshape(_density, [2] * (2 * self.n_wires))  # NOT moved to device!

# In QuantumDevice.__init__ (correct behavior):
_state = torch.reshape(_state, [2] * self.n_wires).to(self.device)  # Moved to device

This caused:

• model.load_state_dict() to fail with device mismatch errors
• GPU-based noise simulations to fail unexpectedly
• State dicts saved from GPU models couldn't be loaded properly

Solution

Added .to(self.device) to the density tensor initialization, consistent with QuantumDevice.

Test plan

• Verify NoiseDevice buffers are on correct device after initialization
• Test save/load with GPU models using NoiseDevice

Fixes #210

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