[torchcodec] fix simple decoder iteration (#59)

Summary:
Pull Request resolved: #59

The previous iterable and iterator implementation had a bug, demonstrated by the test modified in this diff. The problem:

1. We were using the `SimpleVideoDecoder` as its own iterator object by directly implementing `__iter__()` and `__next__()`.
2. In `__next__()`, we were calling a core library function, `get_next_frame()`, that returned the next frame to be decoded, and advanced the internal state of the C++ decoder.
3. But we were not *initializing* the iterator.

Because of the points above, for-based iteration only worked as expected on a freshly-created `SimpleVideoDecoder` object.

The simplest fix is to just remove the implementations of `__iter__()` and `__next__()`. Because it implements `__len__()` and `__getitem__()`, a `SimpleVideoDecoder` is a Python sequence. Python sequences are automatically iterable through `__len__()` and `__getitem__()`. See: https://docs.python.org/3/glossary.html#term-iterable

Differential Revision: D59309882

src/torchcodec/decoders/_simple_video_decoder.py

@@ -67,17 +67,6 @@ def __getitem__(self, key: Union[int, slice]) -> torch.Tensor:
             f"Unsupported key type: {type(key)}. Supported types are int and slice."
         )
 
-    def __iter__(self) -> "SimpleVideoDecoder":
-        return self
-
-    def __next__(self) -> torch.Tensor:
-        # TODO: We should distinguish between expected end-of-file and unexpected
-        # runtime error.
-        try:
-            return core.get_next_frame(self._decoder)
-        except RuntimeError:
-            raise StopIteration()
-
 
 def _get_and_validate_stream_metadata(decoder: torch.Tensor) -> core.StreamMetadata:
     video_metadata = core.get_video_metadata(decoder)

test/decoders/simple_video_decoder_test.py

@@ -201,14 +201,26 @@ def test_next(self):
 
         ref_frame0 = NASA_VIDEO.get_tensor_by_index(0)
         ref_frame1 = NASA_VIDEO.get_tensor_by_index(1)
+        ref_frame9 = NASA_VIDEO.get_tensor_by_index(9)
+        ref_frame35 = NASA_VIDEO.get_tensor_by_index(35)
         ref_frame180 = NASA_VIDEO.get_tensor_by_name("time6.000000")
         ref_frame_last = NASA_VIDEO.get_tensor_by_name("time12.979633")
 
+        # Access an arbitrary frame to make sure that the later iteration
+        # still works as expected. The underlying C++ decoder object is
+        # actually stateful, and accessing a frame will move its internal
+        # cursor.
+        assert_tensor_equal(ref_frame35, decoder[35])
+
         for i, frame in enumerate(decoder):
             if i == 0:
                 assert_tensor_equal(ref_frame0, frame)
             elif i == 1:
                 assert_tensor_equal(ref_frame1, frame)
+            elif i == 9:
+                assert_tensor_equal(ref_frame9, frame)
+            elif i == 35:
+                assert_tensor_equal(ref_frame35, frame)
             elif i == 180:
                 assert_tensor_equal(ref_frame180, frame)
             elif i == 389: