Fix WanVAE_.encode() to return both mu and log_var

[Mr-Neutr0n]

Summary

• Fixed WanVAE_.encode() to return both mu and log_var as expected by forward()
• Made scale parameter optional (defaults to None) in both encode() and decode() methods

Problem

The forward() method calls self.encode(x) expecting two return values (mu, log_var):

mu, log_var = self.encode(x)

However, encode() was only returning mu, causing a ValueError: not enough values to unpack.

Additionally, encode(x, scale) required the scale parameter, but forward() didn't pass it.

Changes

1. Modified encode() to return (mu, log_var) tuple
2. Made scale parameter optional with scale=None default
3. Made scale parameter optional in decode() as well for consistency
4. Updated WanVAE.encode() wrapper to extract only mu with [0] index

Related Issue

Fixes #556

Test Plan

• forward() now works without passing scale (uses no scaling when scale=None)
• Explicit scale parameter still works as before
• WanVAE wrapper continues to work unchanged

[Mr-Neutr0n]

bump on this — the encode() method was only returning mu and dropping log_var, which breaks anyone trying to use the VAE's full latent distribution. would appreciate a review