ggml : avoid multiply by D in GGML_OP_SSM_SCAN

This makes the weight buft detection in src/llama.cpp simpler.

* convert : transpose Mamba-2 A, D and reshape SSM_NORM

This breaks existing conversions of Mamba-2 models
to avoid some reshapes.

Not sure if it's a good idea,
but it makes the graph slightly cleaner.

* llama : more appropriate SSM_SCAN and SSM_CONV buft support checks

ggml/include/ggml.h

@@ -1828,7 +1828,6 @@ extern "C" {
             struct ggml_tensor  * A,
             struct ggml_tensor  * B,
             struct ggml_tensor  * C,
-            struct ggml_tensor  * D,
             struct ggml_tensor  * ids);
 
     // partition into non-overlapping windows with padding if needed