[Performance] Remove redundant clone() calls in cutlass_mla (vllm-project#24891)

Author: alexm-redhat

vllm/v1/attention/backends/mla/cutlass_mla.py

@@ -210,9 +210,14 @@ def _sm100_cutlass_mla_decode(
             sm_scale,
             num_kv_splits,
         )
-        returned_lse = lse[:, :H].contiguous(
-        ) if self.need_to_return_lse_for_decode else lse
-        return out[:, :H].contiguous(), returned_lse
+
+        if H < MAX_HEADS:
+            # Extract the subsets of the outputs
+            returned_lse = lse[:, :H].contiguous(
+            ) if self.need_to_return_lse_for_decode else lse
+            out = out[:, :H]
+
+        return out, returned_lse
 
     def _sm100_forward_decode(
         self,
@@ -228,11 +233,6 @@ def _sm100_forward_decode(
         self._workspace.ensure_size(attn_metadata, self._num_kv_splits)
 
         # Run MLA
-        # Clone q_nope and q_pe to make sure strides computation is correct.
-        # TODO: Check if we really need it
-        q_nope = q_nope.clone()
-        q_pe = q_pe.clone()
-
         o, lse = self._sm100_cutlass_mla_decode(
             q_nope,
             q_pe,