model.inference() should take other inputs instead of only file input
**Is your feature request related to a problem? Please describe.**

I trained a pre-trained model Bert on the Squad dataset for question_answer but to get the answer for a single question there is nothing that **model.inference** provides me as it only accepts a file. 
```
model = BERTQAModel.restore_from(config.model.nemo_path)

eval_device = [config.trainer.devices[0]] if isinstance(config.trainer.devices, list) else 1
model.trainer = pl.Trainer(
    devices=eval_device,
    accelerator=config.trainer.accelerator,
    precision=16,
    logger=False,
)

all_preds, all_nbest = model.inference(
    config.model.test_ds.file, # file_name
    num_samples=10, # setting to -1 will use all samples for inference
)
```

**Describe the solution you'd like**
So, if we can pass the question string and model infer on the question string to return us the answer, it would be great.

```
answer = model.inference("question") 
# here question can be a string on which the model can infer to give us the result or it also can be a JSON obj.
```