Update HW decoding tutorial and add notes about unseekable object (#2408)

Summary:
https://output.circle-artifacts.com/output/job/75187a52-b0d8-4cac-89f3-24e10889a36a/artifacts/0/docs/hw_acceleration_tutorial.html

1. Update HW decoding tutorial to include file-like object
1. Add note about unseekable object int streaming API tutorial

Pull Request resolved: https://github.com/pytorch/audio/pull/2408

Reviewed By: hwangjeff

Differential Revision: D36632702

Pulled By: mthrok

fbshipit-source-id: 17be2fb8528cb1d2d1ee11901b6a95c512466feb

examples/tutorials/streaming_api_tutorial.py

@@ -188,7 +188,7 @@ VIDEO_URL = f"{base_url}/stream-api/NASAs_Most_Scientifically_Complex_Space_Obse
 #
 # .. code::
 #
-#    class Wrapper:
+#    class UnseekableWrapper:
 #        def __init__(self, obj):
 #            self.obj = obj
 #
@@ -200,14 +200,24 @@ VIDEO_URL = f"{base_url}/stream-api/NASAs_Most_Scientifically_Complex_Space_Obse
 #    import requests
 #
 #    response = requests.get("https://example.com/video.mp4", stream=True)
-#    s = StreamReader(Wrapper(response.raw))
+#    s = StreamReader(UnseekableWrapper(response.raw))
 #
 # .. code::
 #
 #    import boto3
 #
 #    response = boto3.client("s3").get_object(Bucket="my_bucket", Key="key")
-#    s = StreamReader(Wrapper(response["Body"]))
+#    s = StreamReader(UnseekableWrapper(response["Body"]))
+#
+# .. note::
+#
+#    When using an unseekable file-like object, the source media has to be
+#    streamable.
+#    For example, a valid MP4-formatted object can have its metadata either
+#    at the beginning or at the end of the media data.
+#    Those with metadata at the beginning can be opened without method
+#    `seek`, but those with metadata at the end cannot be opened
+#    without `seek`.
 #
 
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