Add _unsafe_set_level for torchdim (#924)

[torchdim](https://github.com/facebookresearch/torchdim) reuses BatchTensorImpl objects
where it can across batched invocations, but needs to patch their levels to match
what _vmap_increment_nesting returns.

[ghstack-poisoned]

Author: zdevito

functorch/csrc/BatchedTensorImpl.h

@@ -78,7 +78,9 @@ struct BatchedTensorImpl : public c10::TensorImpl {
 #endif
 
   void refreshTensorMetadata();
-
+  void _unsafe_set_level(int64_t level) {
+    level_ = level;
+  }
  private:
   // see NOTE: [BatchedTensorImpl levels invariant]
   void checkInvariants() const;