fix: don't re-download encoder weights when loading a pretrained model

[cycsmail]

Fixes #957.

When you reload a model with from_pretrained, the encoder weights are already stored in the checkpoint, but the saved config still has encoder_weights set (e.g. "imagenet"), so smp went and re-downloaded the encoder's pretrained weights anyway. That makes loading fail offline / from a purely local path.

This skips the download when the config records encoder_weights, falling back to None so the checkpoint weights are used as-is. An explicit encoder_weights=... passed to from_pretrained still takes precedence via setdefault.

Added a test that asserts no encoder download happens on reload and that the restored weights match the saved model.