Qwen MoE model. (#1960)

* Qwen MoE model.

* Add the MoE model to the example.

* Fix the scaling.

* Readme updates.

* Readme tweaks.

candle-examples/examples/qwen/README.md

@@ -0,0 +1,27 @@
+# candle-qwen: large language model series from Alibaba Cloud
+
+Qwen 1.5 is a series of large language models that provide strong performances
+on English and Chinese.
+
+- [Blog post](https://qwenlm.github.io/blog/qwen1.5/) introducing Qwen1.5.
+- [Model card](https://huggingface.co/Qwen/Qwen1.5-0.5B) on the HuggingFace Hub.
+- [Blog post](https://qwenlm.github.io/blog/qwen-moe/) for the
+  mixture-of-experts (MoE) variant.
+
+## Running the example
+
+```bash
+$ cargo run --example qwen --release  -- --prompt "Hello there "
+```
+
+Various model sizes are available via the `--model` argument, including the MoE
+variant.
+
+```bash
+$ cargo run --example qwen --release  -- --prompt "Hello there " --model moe-a2.7b --prompt 'def print_prime(n: int): '
+def print_prime(n: int):  # n is the number of primes to be printed
+    for i in range(2, n + 1):
+        if all(i % j != 0 for j in range(2, i)):
+            print(i)
+```
+