Metavoice - first cut (#1717)

* Add the metavoice transformer.

* Sketch the speaker-encoder module.

* Adding to the metavoice model.

* Start adding the metavoice example.

* Get some logits out.

* Load the second stage model.

* Get the second step to run.

* Tweak the example.

* Add encodec tilting.

* Glue the different bits together.

* Fix a shape issue.

* Use a constant.

* BPE tokenization.

* Add a warning.

candle-examples/examples/metavoice/README.md

@@ -0,0 +1,18 @@
+# candle-metavoice
+
+MetaVoice-1B is a text-to-speech model trained on 100K hours of speech, more
+details on the [model
+card](https://huggingface.co/metavoiceio/metavoice-1B-v0.1).
+
+Note that the current candle implementation suffers from some limitations as of
+2024-03-02:
+- The speaker embeddings are hardcoded.
+- The generated audio file quality is weaker than the Python implementation,
+  probably because of some implementation discrepancies.
+
+## Run an example
+
+```bash
+cargo run --example metavoice --release -- \\
+  --prompt "This is a demo of text to speech by MetaVoice-1B, an open-source foundational audio model."
+```