[fix] Fix the activation checkpointing when using SwiGLUPackedFusedOp

According to the docs (https://pytorch.org/docs/stable/autograd.html#torch.autograd.Function) forward() method should not be called directly, apply() method have to be used instead.
After removing forward call, activation checkpointing starts working. (alternative variant #2)

xformers/csrc/swiglu/swiglu_packedw.cpp

@@ -211,7 +211,12 @@ at::Tensor swiglu_packedw_cuda(
     const std::optional<at::Tensor> b1b2,
     const at::Tensor w3,
     const std::optional<at::Tensor> b3) {
+  if (torch::GradMode::is_enabled()) {
     return SwiGLUPackedWeights::apply(x, w1w2, b1b2, w3, b3);
+  } else {
+    return SwiGLUPackedWeights::forward(
+        /* ctx */ nullptr, x, w1w2, b1b2, w3, b3);
+  }
 }
 } // namespace