fix: overwrite to free previous chunk from the memory

Co-authored-by: thomas chaton <[email protected]>

src/litdata/streaming/item_loader.py

@@ -175,7 +175,8 @@ def _load_encrypted_data(
             else:
                 encrypted_data = fp.read()
                 decrypted_data = encryption.decrypt(encrypted_data)  # type: ignore
-                self._decrypted_chunks[chunk_index] = decrypted_data
+               # This would enable us to free the previous chunk from the memory.
+                self._decrypted_chunks = {chunk_index: decrypted_data}
             fp = BytesIO(decrypted_data)
 
         data = self._load_data(fp, offset)