load_vectors fix for new version of touchtext

[Aegeaner]

 ✗ python test/example.py                 
 Generating train, dev, test splits
 Building vocabulary
 Traceback (most recent call last):
   File "test/example.py", line 16, in <module>
     inputs.vocab.load_vectors(wv_type='glove.840B', wv_dim=300)
 TypeError: load_vectors() missing 1 required positional argument: 'vectors'

[salesforce-cla[bot]]

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[Aegeaner]

cla signed.