Indicate num_ffmpeg_threads is only relevant on CPU

Author: NicolasHug

src/torchcodec/decoders/_video_decoder.py

@@ -43,11 +43,11 @@ class VideoDecoder:
                 cheap no-copy operation that allows these frames to be
                 transformed using the `torchvision transforms
                 <https://pytorch.org/vision/stable/transforms.html>`_.
-        num_ffmpeg_threads (int, optional): The number of threads to use for decoding.
+        num_ffmpeg_threads (int, optional): The number of threads to use for CPU decoding.
             Use 1 for single-threaded decoding which may be best if you are running multiple
             instances of ``VideoDecoder`` in parallel. Use a higher number for multi-threaded
             decoding which is best if you are running a single instance of ``VideoDecoder``.
-            Default: 1.
+            Default: 1. Ignored if ``device`` is not "cpu".
         device (str or torch.device, optional): The device to use for decoding. Default: "cpu".