Add GGML_METAL_FUSE_GDN_CACHE to the fusion table: when the gated_delta_net
kernel is followed by a cpy that scatters its recurrent state snapshots into
the KV cache, the kernel writes the snapshots straight into the cache buffer
and the trailing cpy is elided.
The gdn output has other consumers (the attn scores view), so unlike the
elision-chain patterns this is not a simple chain: a 'raw' flag on the fusion
pattern skips the generic chain/shape and ggml_can_fuse_subgraph_ext checks,
making the pattern-specific check callback the sole validator. Packing
(ggml_metal_fuse_max) now matches on the same view-transparent node sequence
that the compute phase uses, so the gdn + cache cpy group is packed along with
any intermediate views and stays adjacent through the reorder.
The fused cpy is a view consumer of the gdn (it writes the cache directly),
so its mem-range is skipped in the encoder; the skip is restricted to CPY
nodes consuming the previous fused node through a view so other fusions are
unaffected.
Add test_gated_delta_net_cache_fusion and register 5 cases.
Assisted-by: pi:llama.cpp/DeepSeek-V4-Flash-0731
Georgi Gerganov
f2bc4dfa87