whisper : use vulkan as gpu backend when available (whisper/2302)

* ggml: use vulkan as gpu backend when available

Signed-off-by: Matt Stephenson <mstephenson6@users.noreply.github.com>

* whisper: enable using vk as default buffer type

Signed-off-by: Matt Stephenson <mstephenson6@users.noreply.github.com>

---------

Signed-off-by: Matt Stephenson <mstephenson6@users.noreply.github.com>

ggml/src/ggml-vulkan.cpp

@@ -2108,9 +2108,9 @@ void ggml_vk_instance_init() {
 }
 
 static void ggml_vk_init(ggml_backend_vk_context * ctx, size_t idx) {
-    GGML_ASSERT(idx < vk_instance.device_indices.size());
     VK_LOG_DEBUG("ggml_vk_init(" << ctx->name << ", " << idx << ")");
     ggml_vk_instance_init();
+    GGML_ASSERT(idx < vk_instance.device_indices.size());
 
     ctx->name = GGML_VK_NAME + std::to_string(idx);