f-string: empty expression not allowed

In Deep learning EEG_model testing_DL notebook
```python
output_filename = f'Encoder_regressor_CROSSVALIDATION_{}'
```
```python
  File "/var/folders/81/y875xrcn5_jf7gqry_8s1jv40000gn/T/ipykernel_49880/379230724.py", line 10
    output_filename = f'Encoder_regressor_CROSSVALIDATION_{}'
                      ^
SyntaxError: f-string: empty expression not allowed
```