[HacktoberFest][Missing Docstrings] Autoencoder `encode`

[patrickvonplaten]

This feature request is included in this years Diffusers' Hacktoberfest 👨‍💻!

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Issue

The encode method of AutoencoderKL is completely missing docstrings, can you add them?

[freespirit]

I am solving this issue in #5239