Repository: Unbabel/COMET
Issue: ReferencelessRegression cannot be instantiated with the default args
Root cause: Validation Gap

## 🐛 Bug

`ReferencelessRegression` cannot be instantiated with the default args -- it looks to me like train and valid filepaths default to None and then there's an attempt to iterate over the None.

### To Reproduce 

1. Failing example with the default args:

```python
from comet.models import ReferencelessRegression

model = ReferencelessRegression()
```

Output:
```bash
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[1], line 3
      1 from comet.models import ReferencelessRegression
----> 3 model = ReferencelessRegression()

File ~/Library/Caches/pypoetry/virtualenvs/venv/lib/python3.12/site-packages/comet/models/regression/referenceless.py:97, in ReferencelessRegression.__init__(self, nr_frozen_epochs, keep_embeddings_frozen, optimizer, warmup_steps, encoder_learning_rate, learning_rate, layerwise_decay, encoder_model, pretrained_model, pool, layer, layer_transformation, layer_norm, loss, dropout, batch_size, train_data, validation_data, hidden_sizes, activations, final_activation, load_pretrained_weights, local_files_only)
     71 def __init__(
     72     self,
     73     nr_frozen_epochs: Union[float, int] = 0.3,
   (...)
     95     local_files_only: bool = False,
     96 ) -> None:
---> 97     super(RegressionMetric, self).__init__(
     98         nr_frozen_epochs=nr_frozen_epochs,
     99         keep_embeddings_frozen=keep_embeddings_frozen,
    100         optimizer=optimizer,
    101         warmup_steps=warmup_steps,
    102         encoder_learning_rate=encoder_learning_rate,
    103         learning_rate=learning_rate,
    104         layerwise_decay=layerwise_decay,
    105         encoder_model=encoder_model,
    106         pretrained_model=pretrained_model,
    107         pool=pool,
    108         layer=layer,
    109         layer_transformation=layer_transformation,
    110         layer_norm=layer_norm,
    111         loss=loss,
    112         dropout=dropout,
    113         batch_size=batch_size,
    114         train_data=train_data,
    115         validation_data=validation_data,
    116         class_identifier="referenceless_regression_metric",
    117         load_pretrained_weights=load_pretrained_weights,
    118         local_files_only=local_files_only,
    119     )
    120     self.save_hyperparameters()
    121     self.estimator = FeedForward(
    122         in_dim=self.encoder.output_units * 4,
    123         hidden_sizes=self.hparams.hidden_sizes,
   (...)
    127         out_dim=1,
    128     )

File ~/Library/Caches/pypoetry/virtualenvs/venv/lib/python3.12/site-packages/comet/models/base.py:152, in CometModel.__init__(self, nr_frozen_epochs, keep_embeddings_frozen, optimizer, warmup_steps, encoder_learning_rate, learning_rate, layerwise_decay, encoder_model, pretrained_model, pool, layer, layer_transformation, layer_norm, loss, dropout, batch_size, train_data, validation_data, class_identifier, load_pretrained_weights, local_files_only)
    149 self.pool = pool
    151 # If not defined here, metrics will not live in the same device as our model.
--> 152 self.init_metrics()

File ~/Library/Caches/pypoetry/virtualenvs/venv/lib/python3.12/site-packages/comet/models/regression/regression_metric.py:137, in RegressionMetric.init_metrics(self)
    134 """Initializes train/validation metrics."""
    135 self.train_metrics = RegressionMetrics(prefix="train")
    136 self.val_metrics = nn.ModuleList(
--> 137     [RegressionMetrics(prefix=d) for d in self.hparams.validation_data]
    138 )

TypeError: 'NoneType' object is not iterable
```

2. Working example with training and valid filepaths set to empty lists:

```python
from comet.models import ReferencelessRegression

model = ReferencelessRegression(train_data=[], validation_data=[])
```

Output:
```bash
Encoder model frozen.
```

### Environment
OS: [Mac Apple M1, Sequoia 15.1.1]
<br>
Packaging [poetry@1.8.4]
<br>
Version [2.2.4]