Revert ultralytics-thop to optional (#13290)

ultralytics/nn/tasks.py

@@ -4,7 +4,6 @@ import contextlib
 from copy import deepcopy
 from pathlib import Path
 
-import thop
 import torch
 import torch.nn as nn
 
@@ -66,6 +65,11 @@ from ultralytics.utils.torch_utils import (
     time_sync,
 )
 
+try:
+    import thop
+except ImportError:
+    thop = None
+
 
 class BaseModel(nn.Module):
     """The BaseModel class serves as a base class for all the models in the Ultralytics YOLO family."""
@@ -153,7 +157,7 @@ class BaseModel(nn.Module):
             None
         """
         c = m == self.model[-1] and isinstance(x, list)  # is final layer list, copy input as inplace fix
-        flops = thop.profile(m, inputs=[x.copy() if c else x], verbose=False)[0] / 1e9 * 2  # GFLOPs
+        flops = thop.profile(m, inputs=[x.copy() if c else x], verbose=False)[0] / 1e9 * 2 if thop else 0  # GFLOPs
         t = time_sync()
         for _ in range(10):
             m(x.copy() if c else x)