Enable ffmpeg prototyep unit test (#2261)

Summary:
Pull Request resolved: https://github.com/pytorch/audio/pull/2261

Enables prototype ffmpeg io tests in fbcode.

Reviewed By: nateanl

Differential Revision: D33698353

fbshipit-source-id: 61de997c564135e677cd68e34fd7cc5dc0c5e036

test/torchaudio_unittest/common_utils/case_utils.py

@@ -98,6 +98,8 @@ class TorchaudioTestCase(TestBaseMixin, PytorchTestCase):
 
 
 def is_ffmpeg_available():
+    if _eval_env("TORCHAUDIO_TEST_IN_FBCODE", default=False):
+        return True
     try:
         return torchaudio._extension._load_lib("libtorchaudio_ffmpeg")
     except Exception: