Make it optional to build CUDA extension for SAM 2; also fallback to all available kernels if Flash Attention fails (#155)

In this PR, we make it optional to build the SAM 2 CUDA extension, in observation that many users encounter difficulties with the CUDA compilation step.
1. During installation, we catch build errors and print a warning message. We also allow explicitly turning off the CUDA extension building with `SAM2_BUILD_CUDA=0`.
2. At runtime, we catch CUDA kernel errors from connected components and print a warning on skipping the post processing step.

We also fall back to the all available kernels if the Flash Attention kernel fails.

sam2/utils/misc.py

@@ -220,10 +220,24 @@ def fill_holes_in_mask_scores(mask, max_area):
     # Holes are those connected components in background with area <= self.max_area
     # (background regions are those with mask scores <= 0)
     assert max_area > 0, "max_area must be positive"
-    labels, areas = get_connected_components(mask <= 0)
-    is_hole = (labels > 0) & (areas <= max_area)
-    # We fill holes with a small positive mask score (0.1) to change them to foreground.
-    mask = torch.where(is_hole, 0.1, mask)
+
+    input_mask = mask
+    try:
+        labels, areas = get_connected_components(mask <= 0)
+        is_hole = (labels > 0) & (areas <= max_area)
+        # We fill holes with a small positive mask score (0.1) to change them to foreground.
+        mask = torch.where(is_hole, 0.1, mask)
+    except Exception as e:
+        # Skip the post-processing step on removing small holes if the CUDA kernel fails
+        warnings.warn(
+            f"{e}\n\nSkipping the post-processing step due to the error above. "
+            "Consider building SAM 2 with CUDA extension to enable post-processing (see "
+            "https://github.com/facebookresearch/segment-anything-2/blob/main/INSTALL.md).",
+            category=UserWarning,
+            stacklevel=2,
+        )
+        mask = input_mask
+
     return mask