Disabled work remap on low sequence length

[tarik-sarac]

Disabled work remap on short sequence lengths since it doesn't have any additional advantage on lower sequences and additional overhead of work remap slows down short sequences further.

• Added a new helper function bwd_sorted_remap_min_col_tiles() in flash_api.cpp that reads the minimum column tiles threshold from the BACK_LITE_BWD_SORT_REMAP_MIN_COL_TILES environment variable, enabling runtime configuration of when sorted remapping is applied.
• Updated the backward pass logic to conditionally enable sorted column remapping only when params.num_col_tiles meets or exceeds the configured threshold; otherwise, disables remapping by setting params.work_remap to nullptr.