Add directory validation, empty dataset check and move metrics to module level in utils.py

[agentksimha]

Changes made to RenAIssance_Transformer_OCR_Utsav_Rai/code/utils.py:

1. Added image_dir and text_dir validation in SpanishDocumentsDataset.init
   Both directories were never validated before use. An invalid path causes os.listdir to throw a generic FileNotFoundError with no indication of which argument was wrong. Added explicit FileNotFoundError checks for both with descriptive messages.

2. Added empty dataset check in SpanishDocumentsDataset.init
   If image_dir exists but contains no .jpg files, self.filenames is empty and the DataLoader silently produces nothing during training. Added a ValueError check after scanning for .jpg files.

3. Added image_path validation in generate_text_from_image_segment
   The function had no path existence check before Image.open. A missing file would be caught by the generic except Exception block, producing a vague error message. Added an explicit FileNotFoundError check before the try block so the failure is immediately actionable.

4. Moved load_metric calls to module level
   load_metric was called three times inside compute_metrics, meaning cer, wer and bleu metrics were loaded from disk on every evaluation step during training. Moved all three to module-level constants so they are loaded once at import time and reused across all calls.