fix: Repair import error for legacy TS testing

- Wrap Dynamo import in version check
- Temporarily enable Legacy TS testing on CI

Author: gs-olive

.circleci/config.yml

@@ -1597,3 +1597,22 @@ workflows:
           trt-version-short: << pipeline.parameters.trt-version-short >>
           cudnn-version: << pipeline.parameters.cudnn-version >>
           python-version: << pipeline.parameters.python-version >>
+
+      # Temporary, for CI run
+      - build-x86_64-linux:
+          name: build-x86_64-linux-legacy
+          torch-build: << pipeline.parameters.torch-build-legacy >>
+          torchvision-build: << pipeline.parameters.torchvision-build-legacy >>
+          torch-build-index: << pipeline.parameters.torch-build-index-legacy >>
+          python-version: << pipeline.parameters.python-version >>
+          legacy: << pipeline.parameters.enable-legacy >>
+
+      - test-py-ts-x86_64-linux:
+          name: test-py-ts-x86_64-linux-legacy
+          torch-build: << pipeline.parameters.torch-build-legacy >>
+          torchvision-build: << pipeline.parameters.torchvision-build-legacy >>
+          torch-build-index: << pipeline.parameters.torch-build-index-legacy >>
+          trt-version-long: << pipeline.parameters.trt-version-long >>
+          python-version: << pipeline.parameters.python-version >>
+          requires:
+            - build-x86_64-linux-legacy

py/torch_tensorrt/dynamo/backend/backends.py

@@ -2,7 +2,9 @@
 from typing import Sequence
 import torch
 from functools import partial
-import torch._dynamo as td
+
+if not torch.__version__.startswith("1"):
+    import torch._dynamo as td
 
 from torch_tensorrt.dynamo.backend._settings import CompilationSettings
 from torch_tensorrt.dynamo.backend.lowering._decompositions import (