Complete the mixformer implementation. (#930)

* Complete the mixformers implementation.

* Tweak the attention.

* Add the phi-1.5 example.

* Improve the phi example.

* Bugfix.

* Get the phi example to work.

candle-examples/examples/phi/README.md

@@ -0,0 +1,23 @@
+# candle-starcoder: code generation model
+
+[phi-1.5](https://huggingface.co/microsoft/phi-1_5).
+
+## Running some example
+
+```bash
+$ cargo run --example phi --release -- --prompt "def print_prime(n): "
+
+def print_prime(n): 
+    print("Printing prime numbers")
+    for i in range(2, n+1):
+        if is_prime(i):
+            print(i)
+
+def is_prime(n):
+    if n <= 1:
+        return False
+    for i in range(2, int(math.sqrt(n))+1):
+        if n % i == 0:
+            return False
+    return True
+```