Make SHAP & LIME black box

[Logiquo]

Allow SHAP & LIME to be turely black-box when use_embedding=False, will keep the behaviour the same when use_embedding=True

Fix #844

[Logiquo]

Test running on server.

[Logiquo]

Result

{
   "random":{
      "comp":0.3097134712197836,
      "suff":0.28804294100286915
   },
   "shap (emb)":{
      "comp":0.5698760058365616,
      "suff":0.09969445264113008
   },
   "shap":{
      "comp":0.5698760058365616,
      "suff":0.09969445264113008
   },
   "lime (emb)":{
      "comp":0.19678775907731524,
      "suff":0.37132484801844057
   },
   "lime":{
      "comp":0.2896784844936109,
      "suff":0.3094283782137448
   }
}

[jhnwu3]

@Logiquo purely just readability reasons for code. Can we rename num_tokens to vocab_size?

Num_tokens can often be conflated with sequence length in my mind. Of course, the "size" attribute can also be problematic here haha. So we may need to reconsider the naming of some of these APIs.