qwen4exp: fix seq_cp, block position keying, mtmd input, cuda abort, add tests (#27941)

* qwen4exp: follow up fixes

* -kvu NaN collapse fix

Assisted-by: Claude

* indexer cache ext.x/ext.y restore fix

Assisted-by: Claude

* kv-cells: rename seq_set to seq_get_all

seq_get is already taken by the single-id getter, so the suggested name
cannot be overloaded on return type alone.

Assisted-by: Claude

* memory-hybrid-idx: implement set_input_qsa on the memory class

The context held the whole implementation, where the pattern elsewhere is a
thin context forwarding to the memory class, as llama_kv_cache_context does
for set_input_kq_mask. The body reads no context state, so it moves unchanged
and the context keeps a forwarder.

Also shortens the seq_get_all comment as suggested.

* tests: check that a sequence state survives a save/restore round-trip

Saves seq 0, erases it, restores the blob and saves again, requiring the two
blobs to match. Compares blobs rather than generated text, which cannot see a
field dropped on the way back in.

Note this passes on master for qwen4exp, so it does not demonstrate the
ext.x/ext.y drop this PR fixes; reaching that needs 2D mrope content.

* tests: give the synthetic qwen4exp a PLE so the state test bites

has_cell_ext() is n_pos_per_embd() > 1 || ple_n_heads > 0, and the indexer
cache sets rope_type = NONE, so without a PLE it serializes no cell ext at
all and the round-trip test cannot see a dropped ext.x/ext.y. With one,
removing the ext_set restore in state_read_meta fails the test: 198 of
335692 bytes differ, first at offset 282092.

Loading such a model needed two fixes:

- the row count of per_layer_token_embd came from require_weight(), which a
  model synthesised from metadata alone has no file to answer. Derive it
  from the head ranges and prefer the file's padded count where there is one.
- the PLE conv history is a row of the recurrent cache, so a PLE on a full
  attention layer dereferenced a null p_l. Reject it at load time instead.

The meta mirror is skipped for qwen4exp. It returned NaN logits before this
fixture carried a PLE, which the nmse check passes since a NaN comparison is
false, and aborts with one. -sm tensor on real devices works.

Assisted-by: Claude

* llama: disable -sm tensor for qwen4exp

test-llama-archs skipped the tensor split for this arch from inside the
test, so the arch still advertised support it does not have. Declare it in
llm_arch_supports_sm_tensor instead and drop the test-side exception; the
existing llm_arch_supports_sm_tensor branch then does the skipping.

Assisted-by: Claude

tests/test-llama-archs.cpp

@@ -254,6 +254,30 @@ static gguf_context_ptr get_gguf_ctx(const llm_arch arch, const bool moe) {
         ms.add_kv(LLM_KV_HYPER_CONNECTION_LOW_RANK, uint32_t(8));
         // without this the QSA layers fall back to dense and go uncovered
         ms.add_kv(LLM_KV_ATTENTION_COMPRESS_RATIOS, std::vector<uint32_t>(n_layer, 4));
+
+        // has_cell_ext() needs ple_n_heads here: the indexer cache serializes no ext without it
+        const uint32_t ple_ngram_size      = 3;
+        const uint32_t ple_heads_per_ngram = 2;
+        const uint32_t ple_n_heads         = (ple_ngram_size - 1)*ple_heads_per_ngram;
+        GGML_ASSERT(n_embd % ple_n_heads == 0);
+        const uint32_t ple_head_dim = n_embd/ple_n_heads;
+
+        std::vector<uint64_t> ple_head_offsets(ple_n_heads);
+        std::vector<uint64_t> ple_head_vocab_sizes(ple_n_heads, n_vocab);
+        for (uint32_t h = 0; h < ple_n_heads; h++) {
+            ple_head_offsets[h] = uint64_t(h)*n_vocab;
+        }
+
+        // the PLE history lives in the recurrent cache, so it must sit on a linear attention layer
+        ms.add_kv(LLM_KV_PLE_LAYERS,                  std::vector<uint32_t>({ 0 }));
+        ms.add_kv(LLM_KV_PLE_NGRAM_SIZE,              ple_ngram_size);
+        ms.add_kv(LLM_KV_PLE_HEADS_PER_NGRAM,         ple_heads_per_ngram);
+        ms.add_kv(LLM_KV_PLE_CONV_KERNEL,             uint32_t(4));
+        ms.add_kv(LLM_KV_PLE_EOS_TOKEN_ID,            uint32_t(0));
+        ms.add_kv(LLM_KV_EMBEDDING_LENGTH_PER_LAYER,  ple_head_dim);
+        ms.add_kv(LLM_KV_PLE_LAYER_MULTIPLIERS,       std::vector<uint64_t>({ 1, 3, 5 }));
+        ms.add_kv(LLM_KV_PLE_HEAD_OFFSETS,            ple_head_offsets);
+        ms.add_kv(LLM_KV_PLE_HEAD_VOCAB_SIZES,        ple_head_vocab_sizes);
     }
 
     // minimax-m3 keeps one indexer head per GQA head; the rest use a fixed 64 to match the fused
@@ -709,6 +733,7 @@ static int test_backends(const llm_arch target_arch, const size_t seed, const in
                 std::string status_nmse      = "\033[1;33mSKIP\033[0m";
                 std::string status_roundtrip = "\033[1;33mSKIP\033[0m";
                 char nmse_str[12] = {0};
+
                 bool skip = !arch_supported(arch) || (dc.split_mode == LLAMA_SPLIT_MODE_TENSOR && dc.devs.empty());
                 if (!skip) {
                     if (logits_cpu.empty()) {