context : allow cache-less context for embeddings (#13108)

* context : allow cache-less context for embeddings

ggml-ci

* context : enable reranking with encode()

ggml-ci

* context : encode() clears embd_seq

ggml-ci

* examples : use llama_encode() when appropriate

ggml-ci

* models : nomic bert moe does not require KV cache

* llama : update comments for llama_decode/llama_encode

ggml-ci

* context : update warning log [no ci]

include/llama.h

@@ -922,14 +922,19 @@ extern "C" {
     // Frees a batch of tokens allocated with llama_batch_init()
     LLAMA_API void llama_batch_free(struct llama_batch batch);
 
-    // Processes a batch of tokens with the ecoder part of the encoder-decoder model.
-    // Stores the encoder output internally for later use by the decoder cross-attention layers.
+    // Process a batch of tokens.
+    // In contrast to llama_decode() - this call does not use KV cache.
+    // For encode-decoder contexts, processes the batch using the encoder.
+    // Can store the encoder output internally for later use by the decoder's cross-attention layers.
     //   0 - success
     // < 0 - error. the KV cache state is restored to the state before this call
     LLAMA_API int32_t llama_encode(
             struct llama_context * ctx,
               struct llama_batch   batch);
 
+    // Process a batch of tokens.
+    // Requires KV cache.
+    // For encode-decoder contexts, processes the batch using the decoder.
     // Positive return values does not mean a fatal error, but rather a warning.
     //   0 - success
     //   1 - could not find a KV slot for the batch (try reducing the size of the batch or increase the context)