Allow missing thop package (#19314)

Co-authored-by: UltralyticsAssistant <web@ultralytics.com>

ultralytics/nn/tasks.py

@@ -7,7 +7,6 @@ import types
 from copy import deepcopy
 from pathlib import Path
 
-import thop
 import torch
 
 from ultralytics.nn.modules import (
@@ -86,6 +85,11 @@ from ultralytics.utils.torch_utils import (
     time_sync,
 )
 
+try:
+    import thop
+except ImportError:
+    thop = None  # conda support without 'ultralytics-thop' installed
+
 
 class BaseModel(torch.nn.Module):
     """The BaseModel class serves as a base class for all the models in the Ultralytics YOLO family."""