[ASR] Fix lexicon free decoder integration with wav2letter

[vineelpratap]

The index of symbols in token dict and word dict should be the same for lexicon free decoder to work. This PR fixes it.

Tested on Librispeech using an n-gram character LM.

BEFORE: (Use LM weight = 4)

 | t h e | o l d h e r s | w h o | h u d | m a r e d | m a n d | h l d | n o t n c e | n | n t h e | l t l | o l | o n | p n c e | m a r y | h l | h u n r o u n d | h e r | b r o t h e r s | n e c e | b u t | s e n | t h e v | e | m p e r e | h o w | t h e | p l s r | h e o w | h n d | t o | r e t u r n | h e | h o l | h m a e |

AFTER: (Use LM weight = 4)

 | t h e | s o l d i e r s | w h o | h a d | c a r e d | f o r | a n d | h a d | n o t i c e d | a n d | t a k e n | t h e | t i t l e | g o l d | y o n | p r i n c e s | m a r y | h a d | h u n g | r o u n d | h e r | b r o t h e r ' s | n e c k | b u t | s e i n g | t h e | f a v o u r | t h e | e m p e r o r | s h o w e d | t h e | p r i s o n e r s | t h e y | n o w | h a s t e n e d | t o | r e t u r n | t h e | h o l y | i m a g e | |