Implement DeepSeek V2 (#2744)

* Add deepseek v2

* Fix

* Remove unused

* Add kv cache

* Remove from cargo.toml

* Fix dtype selection logic

* Fix unnecessary u32->f32->gather->u32

* Remove fromstr impl

* Use local scopes for some clarity

* Typo

* Repeat k_pe

* Chain calls to remove mut

* Actually, remove all muts

* Update readme

candle-examples/examples/deepseekv2/README.md

@@ -0,0 +1,33 @@
+# DeepSeek V2
+
+DeepSeek V2 an MoE model featuring MLA (Multi-Latent Attention). There is a lite (16B) and a full (236B) model.
+
+- Context length of **32k tokens** (Lite model), **128k tokens** (full model)
+- 64 routed experts (Lite model), 160 routed experts (full model)
+
+## Running the example
+
+```bash
+$ cargo run --example deepseekv2 --release --features metal -- --prompt "Recursive fibonacci code in Rust:" --which lite --sample-len 150  
+
+fn fibonacci(n: u32) -> u32 {
+    if n <= 1 {
+        return n;
+    } else {
+        return fibonacci(n - 1) + fibonacci(n - 2);
+    }
+}
+
+## Fibonacci code in Python:
+
+def fibonacci(n):
+    if n <= 1:
+        return n
+    else:
+        return fibonacci(n-1) + fibonacci(n-2)
+
+## Fibonacci code in JavaScript:
+
+function fibonacci(n) {
+    if (n <= 1
+```