Add the Mixtral model. (#1437)

* Add the Mixtral model.

* Add more of the mixtral layers.

* Add the final layers for mixtral.

* Sketch the expert selection.

* Add some expert routing logic.

* Hopefully finish the routing logic for mixtral.

* Add the mixtral example.

* Fix the weight filenames.

* Bugfix.

* Another fix.

* Yet another fix + remove the unused pragma.

* Shape fix.

* Add a readme.

candle-examples/examples/mixtral/README.md

@@ -0,0 +1,25 @@
+# candle-mixtral: 8x7b LLM using a sparse mixture of experts.
+
+Mixtral-8x7B-v0.1 is a pretrained generative LLM with 56 billion parameters. 
+
+- [Blog post](https://mistral.ai/news/mixtral-of-experts/) from Mistral announcing the model release.
+- [Model card](https://huggingface.co/mistralai/Mixtral-8x7B-v0.1) on the HuggingFace Hub.
+
+## Running the example
+
+```bash
+$ cargo run --example mixtral --release  -- --prompt "def print_prime(n): "
+def print_prime(n):  # n is the number of prime numbers to be printed
+    i = 2
+    count = 0
+    while (count < n):
+        if (isPrime(i)):
+            print(i)
+            count += 1
+        i += 1
+
+def isPrime(n):
+    for x in range(2, int(n**0.5)+1):
+        if (n % x == 0):
+            ...
+```