Add root-level requirements.txt for CEBRA pipeline

[Nikhita-14]

Add root-level requirements.txt for CEBRA pipeline

Adds a root-level requirements.txt for the CEBRA pipeline.

Reason:
The README instructs pip install -r requirements.txt, but the only existing requirements file is inside PreprocessingPipeline/.
Having a root-level file allows users to install dependencies directly from the repo root, as intended.

Affected files (previously missing imports without root-level requirements):

• prepare_cebra_input.py
• preprocessing_functions.py
• train_cebra.py
• train_cebra_cut_supervised.py
• train_cebra_cut_unsupervised.py
• cebra-running-notebook-2.ipynb
• eeg_preprocessing.ipynb

Included core dependencies:

• Numerical & EEG processing: numpy, scipy, mne
• Machine learning / deep learning: torch>=2.0, cebra==0.6.0a2, scikit-learn
• Visualization & utilities: matplotlib, seaborn, tqdm

Feedback and suggestions are welcome!