bugfix in XPU legacy_dygraph distributed training: (#47838)

phi::Alloc() complains about missing device_allocator_

paddle/fluid/platform/collective_helper.cc

@@ -353,6 +353,17 @@ BKCLComm* BKCLCommContext::AssignBKCLComm(
     BKCLContext_t comm, int nranks, int rank, int dev_id, int ring_id) {
   std::unique_ptr<XPUDeviceContext> dev_ctx(
       new XPUDeviceContext(XPUPlace(dev_id)));
+  dev_ctx->SetAllocator(paddle::memory::allocation::AllocatorFacade::Instance()
+                            .GetAllocator(XPUPlace(dev_id))
+                            .get());
+  dev_ctx->SetHostAllocator(
+      paddle::memory::allocation::AllocatorFacade::Instance()
+          .GetAllocator(paddle::platform::CPUPlace())
+          .get());
+  dev_ctx->SetZeroAllocator(
+      paddle::memory::allocation::AllocatorFacade::Instance()
+          .GetZeroAllocator(XPUPlace(dev_id))
+          .get());
 
   BKCLCommImpl* c = new BKCLCommImpl;
   c->set_ring_id(ring_id);