Add einops for the Mamba hub kernel

requirements.txt

@@ -11,3 +11,4 @@ kernels>=0.12.0,<0.13     # transformers' own declared range — 0.15.x broke
                           # ZeroGPU build containers have no nvcc; never add
                           # mamba-ssm/causal-conv1d here, the build will fail)
 accelerate                # device placement / efficient loading
+einops                    # required by the kernels-community mamba-ssm kernel