clip: move model cgraphs into their own files (#17965)

* clip: move model cgraphs into their own files

* more explicit enums

* fix linux build

* fix naming

* missing headers

* nits: add comments for contributors

tools/mtmd/clip-impl.h

@@ -1,3 +1,5 @@
+#pragma once
+
 #include "ggml.h"
 #include "gguf.h"
 #include "clip.h"
@@ -134,6 +136,10 @@
 // align x to upper multiple of n
 #define CLIP_ALIGN(x, n) ((((x) + (n) - 1) / (n)) * (n))
 
+// forward declaration
+// TODO: improve this later
+struct clip_ctx;
+
 enum projector_type {
     PROJECTOR_TYPE_MLP,
     PROJECTOR_TYPE_MLP_NORM,