Add the Gemma models. (#1741)

* Add the Gemma models.

* Add the gemma example.

* Adapt the RmsNorm.

* Get the 2b model to work.

* 7b support.

* Use the config head dim.

* Yet another fix.

* Make the matrixes contiguous.

* Also get the 7b model to work.

* And add to the readme.

candle-examples/examples/gemma/README.md

@@ -0,0 +1,27 @@
+# candle-mistral: 2b and 7b LLMs from Google DeepMind
+
+[Gemma](https://ai.google.dev/gemma/docs) is a collection of lightweight open
+models published by Google Deepmind with a 2b and a 7b variant.
+
+In order to use the example below, you have to accept the license on the
+[HuggingFace Hub Gemma repo](https://huggingface.co/google/gemma-7b) and set up
+your access token via the [HuggingFace cli login
+command](https://huggingface.co/docs/huggingface_hub/guides/cli#huggingface-cli-login).
+
+## Running the example
+
+```bash
+$ cargo run --example gemma --release -- --prompt "fn count_primes(max_n: usize)"
+fn count_primes(max_n: usize) -> usize {
+    let mut primes = vec![true; max_n];
+    for i in 2..=max_n {
+        if primes[i] {
+            for j in i * i..max_n {
+                primes[j] = false;
+             }
+         }
+    }
+    primes.len()
+}
+```
+