Fix fairseq

[eginhard]

Copied from coqui-ai#3502 (supersedes coqui-ai#3500 and coqui-ai#3471, fixes coqui-ai#3142, fixes coqui-ai#3361, fixes coqui-ai#3501, fixes coqui-ai#3638):

Fix error on using fairseq model. Variables like dataset and model aren't defined.

So, this is the bug a user faced recently while using the model fairseq, as you can see the user encountered the error: unbound variable 'dataset' in the file manage.py in the method _set_model_item, on inspecting the code, I found that infact, in the condition when fairseq was in 'model_name' there was no declaration of variable dataset as well as the variable model, so I just added them in the first if condition and it solved the issue. I tested it and it worked like charm.

Code:

from TTS.api import TTS

tts = TTS(model_name="tts_models/eng/fairseq/vits", progress_bar=False, gpu=True)
tts.tts_to_file("This is a test.", file_path="output.wav")

Error:

D:\Software\anaconda3\envs\freqtrade310\python.exe C:\Users\uyplayer\Downloads\uig-script_arabic\model.py 
D:\Software\anaconda3\envs\freqtrade310\lib\site-packages\TTS\api.py:70: UserWarning: `gpu` will be deprecated. Please use `tts.to(device)` instead.
  warnings.warn("`gpu` will be deprecated. Please use `tts.to(device)` instead.")
Traceback (most recent call last):
  File "C:\Users\uyplayer\Downloads\uig-script_arabic\model.py", line 5, in <module>
    tts = TTS(model_name="tts_models/eng/fairseq/vits", progress_bar=False, gpu=True)
  File "D:\Software\anaconda3\envs\freqtrade310\lib\site-packages\TTS\api.py", line 74, in __init__
    self.load_tts_model_by_name(model_name, gpu)
  File "D:\Software\anaconda3\envs\freqtrade310\lib\site-packages\TTS\api.py", line 171, in load_tts_model_by_name
    model_path, config_path, vocoder_path, vocoder_config_path, model_dir = self.download_model_by_name(
  File "D:\Software\anaconda3\envs\freqtrade310\lib\site-packages\TTS\api.py", line 129, in download_model_by_name
    model_path, config_path, model_item = self.manager.download_model(model_name)
  File "D:\Software\anaconda3\envs\freqtrade310\lib\site-packages\TTS\utils\manage.py", line 385, in download_model
    model_item, model_full_name, model, md5sum = self._set_model_item(model_name)
  File "D:\Software\anaconda3\envs\freqtrade310\lib\site-packages\TTS\utils\manage.py", line 304, in _set_model_item
    model_full_name = f"{model_type}--{lang}--{dataset}--{model}"
UnboundLocalError: local variable 'dataset' referenced before assignment

Process finished with exit code 1

fixes: coqui-ai#3142 coqui-ai#3361 coqui-ai#3501

Since this is my first PR, It would be really nice if it would get merged, or if there's anything else I can help with please let me know. ThankYou.