Fix facebookresearch#810

- Update the docstring of `predict_torch`

Based on the conversation of facebookresearch#810 the docstring was updated to provide the user a hint how to interpret the dimensions of the outputs.

segment_anything/predictor.py

@@ -202,13 +202,16 @@ def predict_torch(
             instead of a binary mask.
 
         Returns:
-          (torch.Tensor): The output masks in BxCxHxW format, where C is the
-            number of masks, and (H, W) is the original image size.
+          (torch.Tensor): The output masks in BxCxHxW format, where B is the 
+            number of batches, C is the number of masks per batch, and (H, W) is
+            the original image size.
+            The meaning of B depends on the prompt input.
           (torch.Tensor): An array of shape BxC containing the model's
-            predictions for the quality of each mask.
-          (torch.Tensor): An array of shape BxCxHxW, where C is the number
-            of masks and H=W=256. These low res logits can be passed to
-            a subsequent iteration as mask input.
+            predictions for the quality of each mask per batch.
+          (torch.Tensor): An array of shape BxCxHxW, where B is the 
+            number of batches, C is the number of masks per batch and H=W=256.
+            These low res logits can be passed to a subsequent iteration as mask input.
+            The meaning of B depends on the prompt input.
         """
         if not self.is_image_set:
             raise RuntimeError("An image must be set with .set_image(...) before mask prediction.")