Add support for scratch buffers in jax_triton.

This is required to use device-side TMA descriptors.

PiperOrigin-RevId: 735985603

jaxlib/gpu/triton_kernels.cc

@@ -493,15 +493,7 @@ absl::Status KernelCall::Launch(gpuStream_t stream, void** buffers) {
           param.value)));
     }
   }
-  // Triton's kernel ABI expects an additional scratchpad global memory.
-  // For now it is only used for on-device creation of TMA descriptors, which
-  // we do not use yet, so we are just replacing this argument with a null
-  // pointer.
-  // TODO: b/381242007 - Allocate a proper buffer if we want to use
-  // device-side TMA APIs.
-  void* scratch_ptr = nullptr;  // Alive until kernel_.Launch returns.
-  params.push_back(&scratch_ptr);
-
+  params.push_back(buffers++);  // Scratch buffer.
   return kernel_.Launch(stream, grid_, params.data());
 }