🟡 [mimi] separate encoder and decoder (#49577)
* draft
* [mimi] encoder/decoder: nullable halves in MimiConfig, quantizer moved to MimiModel
* [mimi] drop core edits for the quantizer tie, no longer needed
* [mimi] drop the sub_configs TODO
* [mimi] slimmer halves tests
* [mimi] neutral _conv1d_layer_names, so derived codecs inherit the SEANet encoder
* [mimi] record outputs of MimiEncoderModel / MimiDecoderModel with capture_outputs
* [mimi] device-safe length helpers under device_map
* [mimi] comment why the encoder / decoder configs are not sub_configs
