Remove unused variables (ultralytics#8511)

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

ultralytics/utils/plotting.py

@@ -1028,13 +1028,13 @@ def feature_visualization(x, module_type, stage, n=32, save_dir=Path("runs/detec
     for m in ["Detect", "Pose", "Segment"]:
         if m in module_type:
             return
-    batch, channels, height, width = x.shape  # batch, channels, height, width
+    _, channels, height, width = x.shape  # batch, channels, height, width
     if height > 1 and width > 1:
         f = save_dir / f"stage{stage}_{module_type.split('.')[-1]}_features.png"  # filename
 
         blocks = torch.chunk(x[0].cpu(), channels, dim=0)  # select batch index 0, block by channels
         n = min(n, channels)  # number of plots
-        fig, ax = plt.subplots(math.ceil(n / 8), 8, tight_layout=True)  # 8 rows x n/8 cols
+        _, ax = plt.subplots(math.ceil(n / 8), 8, tight_layout=True)  # 8 rows x n/8 cols
         ax = ax.ravel()
         plt.subplots_adjust(wspace=0.05, hspace=0.05)
         for i in range(n):