real_accelerator validation check for both accelerator and deepspeed.…

…accelerator path (microsoft#2685)

accelerator/real_accelerator.py

@@ -1,11 +1,33 @@
-from .abstract_accelerator import DeepSpeedAccelerator
+try:
+    from accelerator.abstract_accelerator import DeepSpeedAccelerator as dsa1
+except ImportError as e:
+    dsa1 = None
+try:
+    from deepspeed.accelerator.abstract_accelerator import DeepSpeedAccelerator as dsa2
+except ImportError as e:
+    dsa2 = None
 
 ds_accelerator = None
 
 
 def _validate_accelerator(accel_obj):
-    assert isinstance(accel_obj, DeepSpeedAccelerator), \
-        f'{accel_obj.__class__.__name__} accelerator is not subclass of DeepSpeedAccelerator'
+    # because abstract_accelerator has different path during
+    # build time (accelerator.abstract_accelerator)
+    # and run time (deepspeed.accelerator.abstract_accelerator)
+    # and extension would import the
+    # run time abstract_accelerator/DeepSpeedAccelerator as its base
+    # class, so we need to compare accel_obj with both base class.
+    # if accel_obj is instance of DeepSpeedAccelerator in one of
+    # accelerator.abstractor_accelerator
+    # or deepspeed.accelerator.abstract_accelerator, consider accel_obj
+    # is a conforming object
+    if not ((dsa1 != None and isinstance(accel_obj,
+                                         dsa1)) or
+            (dsa2 != None and isinstance(accel_obj,
+                                         dsa2))):
+        raise AssertionError(
+            f'{accel_obj.__class__.__name__} accelerator is not subclass of DeepSpeedAccelerator'
+        )
 
     # TODO: turn off is_available test since this breaks tests
     #assert accel_obj.is_available(), \