support len(datamodule)

Let's add support for len(datamodule) so we can get the following:

```python
len(datamodule)

# prints:  
# train_dataloder_1:   200 samples
# train_dataloader_2:  500 samples
# tval_dataloder_1:    200 samples
# val_dataloader_2:    500 samples
# test_dataloder_1:    200 samples
# test_dataloader_2:   500 samples
```
cc @edenafek 