Left padding when encoding questions

In vqa_processed.py, encode_question fails (index out of range) when the padding desired is 'left' and the question length is larger than the maximum length.

A possible fix is to replace the else branch :
```
else: #['pad'] == 'left'
new_k = k + maxlength - len(ex['question_words_UNK'])
ex['question_wids'][new_k] = word_to_wid[w]
ex['seq_length'] = len(ex['question_words_UNK'])
```

With

```
else: # ['pad'] == 'left'
if maxlength < len(ex['question_words_UNK']):
ex['question_wids'][k] = word_to_id[w]
else:
new_k = k + maxlength - len(ex['question_words_UNK'])
ex['question_wids'][new_k] = word_to_id[w]
```