Cast state tensors to BF16 (via training_dtype()) at all DQN network
input sites: compute_loss_internal states/next_states, select_actions_batch,
curiosity module state/next_state, validation batch, and Q-value logging
batch. Non-network tensors (actions, rewards, dones, weights) are left
as F32 since they participate in loss math, not forward passes.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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