Fix import exceptions in tracing.py for older (<1.12) versions of pyt…

…orch

Summary:
X-link: facebookresearch/d2go#362

Pull Request resolved: facebookresearch#4491

Recently landed D35518556 (facebookresearch@36a65a0) / Github: 36a65a0 throws an exception with older versions of PyTorch, due to a missing library for import. This has been reported by multiple members of the PyTorch community at facebookresearch@36a65a0

This change uses `try/except` to check for libraries and set flags on presence/absence to later guard code that would use them.

Differential Revision: D38879134

fbshipit-source-id: 7c16f5cea14f7f19184f8c14f2503ab330e73b14

detectron2/modeling/poolers.py

@@ -221,16 +221,16 @@ def forward(self, x: List[torch.Tensor], box_lists: List[Boxes]):
         num_level_assignments = len(self.level_poolers)
 
         assert_fx_safe(
-            isinstance(x, list) and isinstance(box_lists, list), "Arguments to pooler must be lists"
+            "isinstance(x, list) and isinstance(box_lists, list)", "Arguments to pooler must be lists"
         )
         assert_fx_safe(
-            len(x) == num_level_assignments,
+            "len(x) == num_level_assignments",
             "unequal value, num_level_assignments={}, but x is list of {} Tensors".format(
                 num_level_assignments, len(x)
             ),
         )
         assert_fx_safe(
-            len(box_lists) == x[0].size(0),
+            "len(box_lists) == x[0].size(0)",
             "unequal value, x[0] batch dim 0 is {}, but box_list has length {}".format(
                 x[0].size(0), len(box_lists)
             ),

detectron2/utils/tracing.py

@@ -1,11 +1,23 @@
 import inspect
 from typing import Union
 import torch
-from torch.fx._symbolic_trace import _orig_module_call
-from torch.fx._symbolic_trace import is_fx_tracing as is_fx_tracing_current
 
 from detectron2.utils.env import TORCH_VERSION
 
+try:
+    from torch.fx._symbolic_trace import is_fx_tracing as is_fx_tracing_current
+
+    tracing_current_exists = True
+except ImportError:
+    tracing_current_exists = False
+
+try:
+    from torch.fx._symbolic_trace import _orig_module_call
+
+    tracing_legacy_exists = True
+except ImportError:
+    tracing_legacy_exists = False
+
 
 @torch.jit.ignore
 def is_fx_tracing_legacy() -> bool:
@@ -20,14 +32,18 @@ def is_fx_tracing_legacy() -> bool:
 def is_fx_tracing() -> bool:
     """Returns whether execution is currently in
     Torch FX tracing mode"""
-    if TORCH_VERSION >= (1, 10):
+    if TORCH_VERSION >= (1, 10) and tracing_current_exists:
         return is_fx_tracing_current()
-    else:
+    elif tracing_legacy_exists:
         return is_fx_tracing_legacy()
+    else:
+        # Can't find either current or legacy tracing indication code.
+        # Enabling this assert_fx_safe() call regardless of tracing status.
+        return False
 
 
 @torch.jit.ignore
-def assert_fx_safe(condition: Union[bool, str], message: str):
+def assert_fx_safe(condition: str, message: str):
     """An FX-tracing safe version of assert.
     Avoids erroneous type assertion triggering when types are masked inside
     an fx.proxy.Proxy object during tracing.