metal: SSM kernel improvements (#17876)

* feat: Add a batched version of ssm_conv

This was done using Claude Code. It found a number of optimizations around
how the threads were organized, resulting in a huge performance boost!

Branch: Mamba2SSD

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

* feat: Optimized SSM_SCAN kernel for metal

This used Claude Code and resulted in a modest performance improvement
while maintaining correctness.

Branch: Mamba2SSD

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

* test: Add test-backend-ops perf tests for SSM_CONV

Branch: SSMKernelImprovements

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

* test: Real representitive tests for SSM_CONV

Branch: SSMKernelImprovements

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

* refactor: Use function constant for ssm_conv batch size

Branch: SSMKernelImprovements

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

* test: backend op tests for ssm_scan from granite4 1b-h

Branch: SSMKernelImprovements

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

* style: remove commented out templates

Branch: SSMKernelImprovements

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

* feat: float4 version of ssm_conv_batched

Branch: SSMKernelImprovements

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

* fix: Add missing ggml_metal_cv_free

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

---------

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>
Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

ggml/src/ggml-metal/ggml-metal-impl.h

@@ -77,6 +77,7 @@
 #define FC_MUL_MV                      600
 #define FC_MUL_MM                      700
 #define FC_ROPE                        800
+#define FC_SSM_CONV                    900
 
 // op-specific constants
 #define OP_FLASH_ATTN_EXT_NQPTG 8