Add notes on file structure in Voxceleb1 based datasets (#2776)

Summary:
The file structure of VoxCeleb1 is as follows:
```
root/
└── wav/
    └── speaker_id folders
```
Users who use [Kaldi](https://github.com/kaldi-asr/kaldi/blob/f6f4ccaf213f0fe8b26e633a7dc0c802150626a0/egs/voxceleb/v1/local/make_voxceleb1_v2.pl) to get the VoxCeleb1 dataset have "dev" and "test" folders above "wav" folder. However, in the file lists like https://www.robots.ox.ac.uk/~vgg/data/voxceleb/meta/veri_test.txt or https://www.robots.ox.ac.uk/~vgg/data/voxceleb/meta/iden_split.txt there is not such differentiation. It's not necessary to put the extracted files into separate folders.

This PR adds notes in `VoxCeleb1Identification` and `VoxCeleb1Verification` datasets to inform the file structure to users.

Pull Request resolved: #2776

Reviewed By: carolineechen

Differential Revision: D40483707

Pulled By: nateanl

fbshipit-source-id: ccd1780a72a5b53f0300c2466c3073a293ad7b8d

Author: nateanl

torchaudio/datasets/voxceleb1.py

@@ -135,6 +135,18 @@ class VoxCeleb1Identification(VoxCeleb1):
             (Default: ``"https://www.robots.ox.ac.uk/~vgg/data/voxceleb/meta/iden_split.txt"``)
         download (bool, optional):
             Whether to download the dataset if it is not found at root path. (Default: ``False``).
+
+    Note:
+        The file structure of `VoxCeleb1Identification` dataset is as follows:
+
+        └─ root/
+
+         └─ wav/
+
+         └─ speaker_id folders
+
+        Users who pre-downloaded the ``"vox1_dev_wav.zip"`` and ``"vox1_test_wav.zip"`` files need to move
+        the extracted files into the same ``root`` directory.
     """
 
     def __init__(
@@ -215,6 +227,18 @@ class VoxCeleb1Verification(VoxCeleb1):
             (Default: ``"https://www.robots.ox.ac.uk/~vgg/data/voxceleb/meta/veri_test.txt"``)
         download (bool, optional):
             Whether to download the dataset if it is not found at root path. (Default: ``False``).
+
+    Note:
+        The file structure of `VoxCeleb1Verification` dataset is as follows:
+
+        └─ root/
+
+         └─ wav/
+
+         └─ speaker_id folders
+
+        Users who pre-downloaded the ``"vox1_dev_wav.zip"`` and ``"vox1_test_wav.zip"`` files need to move
+        the extracted files into the same ``root`` directory.
     """
 
     def __init__(self, root: Union[str, Path], meta_url: str = _VERI_TEST_URL, download: bool = False) -> None: