Add preprocessor_config.json

`AutoFeatureExtractor.from_pretrained("facebook/wav2vec2-large")` needs this file to work.
Currently it throws the following error:
`facebook/wav2vec2-large does not appear to have a file named preprocessor_config.json`

PS: I'm not sure if `"return_attention_mask"`needs to be set to true or false. (´facebook/wav2vec2-base´ has it set to false, while ´facebook/wav2vec2-large-lv60´ has it set to true)

preprocessor_config.json

@@ -0,0 +1,8 @@
+{
+  "do_normalize": true,
+  "feature_size": 1,
+  "padding_side": "right",
+  "padding_value": 0.0,
+  "return_attention_mask": false,
+  "sampling_rate": 16000
+}