Add channel-wise gated delta rule custom op (#20913)

[JakeStevens]

Summary:

channel-wise gated DeltaNet / KDA recurrence:

S_t = (I - beta_t k_t k_t^T) Diag(alpha_t) S_{t-1} + beta_t k_t v_t^T
o_t = S_t^T q_t

Unlike the scalar recurrent_gated_delta_rule, the gate is per-key-channel

This lands a straightforward per-token recurrence; later commits optimize it.

Differential Revision: D111952369

[pytorch-bot]

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