llama : DeepSeek V2/V3 MLA implementation (#12801)

* Merged using squash to remove all noise commit messages

* Force flash attention off for `LLM_ARCH_DEEPSEEK2` - embedding too large

* Removed 3 conts (2x RoPE and 1x RMS-norm)

* Changed to use `<cmath>` instead of `<math.h>`

* Reverted removal of the 3 conts

* Used `reshape` in `llm_graph_context::build_attn_mha()`

* Use `k_pe = ggml_reshape`

* Removed the 3 conts again

* Removed the 3D views of `wk_b` and `wv_b`, and just save and 3D in GGUF

* Removed MQA optimisation from `build_attn_mha()` as no gains now

* Simplified `is_mla` branch in `llm_build_deepseek2()`

* Removed `build_attn_mla` and added `nullptr` to all `build_atnn` calls

* Fixed call to `build_attn` in `llm_build_t5_enc`

src/llama-kv-cache.cpp

@@ -27,7 +27,7 @@ bool llama_kv_cache_unified::init(
 
     recurrent = llama_model_is_recurrent(&model);
     v_trans   = !recurrent && !cparams.flash_attn;
-    can_shift = !recurrent && model.arch != LLM_ARCH_DEEPSEEK2; // not supported due to MLA
+    can_shift = !recurrent;
 
     LLAMA_LOG_INFO("%s: kv_size = %d, offload = %d, type_k = '%s', type_v = '%s', n_layer = %d, can_shift = %d\n",
             __func__, kv_size, offload, ggml_type_name(type_k), ggml_type_name(type_v), n_layer, can_shift);