feat: Add decorator to improve legacy support

- Add general-purpose utility for intercepting invalid Torch version
imports and displaying an error message to the user, halting compilation
- Utility automatically parses string inputs to semantic versions using
the `packaging` utility package
- Decorator can accept variable input versions and checks whether the
current version in use in the environment is at least as large as the
minimum specified version
- Display clear error message upon calling the aten tracer when using a
legacy Torch version, for which the Dynamo import is invalid

Author: gs-olive

py/torch_tensorrt/fx/tracer/dispatch_tracer/aten_tracer.py

@@ -9,7 +9,7 @@
     import torch._dynamo as torchdynamo
 
 from torch.fx.passes.infra.pass_base import PassResult
-
+from torch_tensorrt.fx.utils import req_torch_version
 from torch_tensorrt.fx.passes.lower_basic_pass_aten import (
     compose_bmm,
     compose_chunk,
@@ -91,6 +91,7 @@ def setting_python_recursive_limit(limit: int = 10000) -> Generator[None, None,
         sys.setrecursionlimit(default)
 
 
+@req_torch_version("2.0")
 def dynamo_trace(
     f: Callable[..., Value],
     # pyre-ignore
@@ -104,11 +105,6 @@ def dynamo_trace(
     this config option alltogether.  For now, it helps with quick
     experiments with playing around with TorchDynamo
     """
-    if torch.__version__.startswith("1"):
-        raise ValueError(
-            f"The aten tracer requires Torch version >= 2.0. Detected version {torch.__version__}"
-        )
-
     if dynamo_config is None:
         dynamo_config = DynamoConfig()
     with using_config(dynamo_config), setting_python_recursive_limit(2000):
@@ -131,11 +127,13 @@ def dynamo_trace(
             ) from exc
 
 
+@req_torch_version("2.0")
 def trace(f, args, *rest):
     graph_module, guards = dynamo_trace(f, args, True, "symbolic")
     return graph_module, guards
 
 
+@req_torch_version("2.0")
 def opt_trace(f, args, *rest):
     """
     Optimized trace with necessary passes which re-compose some ops or replace some ops

py/torch_tensorrt/fx/utils.py

@@ -1,5 +1,6 @@
 from enum import Enum
-from typing import List
+from typing import List, Callable
+from packaging import version
 
 # @manual=//deeplearning/trt/python:py_tensorrt
 import tensorrt as trt
@@ -104,3 +105,36 @@ def f(*inp):
     mod = run_const_fold(mod)
     mod = replace_op_with_indices(mod)
     return mod
+
+
+def req_torch_version(min_torch_version: str = "2.0"):
+    """
+    Create a decorator which verifies the Torch version installed
+    against a specified version range
+
+    Args:
+        min_torch_version (str): The minimum required Torch version
+        for the decorated function to work properly
+
+    Returns:
+        A decorator which raises a descriptive error message if
+        an unsupported Torch version is used
+    """
+
+    def nested_decorator(f: Callable):
+        def function_wrapper(*args, **kwargs):
+            # Parse minimum and current Torch versions
+            min_version = version.parse(min_torch_version)
+            current_version = version.parse(torch.__version__)
+
+            if current_version < min_version:
+                raise AssertionError(
+                    f"Expected Torch version {min_torch_version} or greater, "
+                    + f"when calling {f}. Detected version {torch.__version__}"
+                )
+            else:
+                return f(*args, **kwargs)
+
+        return function_wrapper
+
+    return nested_decorator