NMT validation examples with inputs
This PR extends the log of NMT models.
It adds input to the translations and ground truths of the logged samples.

Example of updated log (notice `Input: `):

```
[NeMo I 2021-11-15 14:11:28 mt_enc_dec_model:389] Dataset name: Validation, Dataloader index: 0, Translation Examples:
[NeMo I 2021-11-15 14:11:28 mt_enc_dec_model:394]     E̲x̲a̲m̲p̲l̲e̲ ̲0̲:
[NeMo I 2021-11-15 14:11:28 mt_enc_dec_model:395]     Input:        Salto al azul
[NeMo I 2021-11-15 14:11:28 mt_enc_dec_model:396]     Prediction:   iereiereiereieres
 [NeMo I 2021-11-15 14:11:28 mt_enc_dec_model:397]    Ground Truth: Leap into the water
[NeMo I 2021-11-15 14:11:28 mt_enc_dec_model:394]     E̲x̲a̲m̲p̲l̲e̲ ̲1̲:
[NeMo I 2021-11-15 14:11:28 mt_enc_dec_model:395]     Input:        Reacciones
[NeMo I 2021-11-15 14:11:28 mt_enc_dec_model:396]     Prediction:   ristris
[NeMo I 2021-11-15 14:11:28 mt_enc_dec_model:397]     Ground Truth: Reactions
[NeMo I 2021-11-15 14:11:28 mt_enc_dec_model:394]     E̲x̲a̲m̲p̲l̲e̲ ̲2̲:
[NeMo I 2021-11-15 14:11:28 mt_enc_dec_model:395]     Input:        Informalidad
[NeMo I 2021-11-15 14:11:28 mt_enc_dec_model:396]     Prediction:   iereiereiereiere 
[NeMo I 2021-11-15 14:11:28 mt_enc_dec_model:397]     Ground Truth: Informality
```