Fix VAE spatial sharding dynamic calculation bug in Wan pipeline.

Previously, setting `vae_spatial: -1` in the config (intended to trigger dynamic calculation of the VAE spatial sharding axis size) was ineffective because `pyconfig.py` prematurely overrode any `-1` or missing `vae_spatial` value to `1`.

Furthermore, the dynamic calculation formula in `wan_pipeline.py` (`vae_spatial = (2 * total_devices) // dp_size`) was not robust. On single-device runs (where `total_devices=1` and `dp_size=1`) or
  configurations with odd data parallel (DP) sizes, it would calculate a `vae_spatial` value (e.g., 2) that does not divide `total_devices`, failing the mesh validation assertion.