mtmd, server: add "placeholder bitmap" for counting tokens , add */input_tokens API

[ngxson]

Overview

Tokenizing / preprocessing multimodal input is much more CPU-intensive than tokenizing, because it runs on single thread. This can be wasteful if the user just want to count the number of tokens occupied by an image/audio chunk, without actually using the underlay data.

This PR allow create a "placeholder" bitmap that only contains the dimension, no data buffer will be allocated. Preprocessing ops (i.e. image manipulation) will skip processing it

New server APIs are also added to demonstrate this (support counting both tools input tokens and multimodal input tokens)

• /v1/chat/completions/input_tokens
• /v1/responses/input_tokens

In next PRs:

• Skip preprocess audio
• Move std/mean f32 to cgraph
• Update for places where process_mtmd_prompt is being used for counting tokens

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: no

[ngxson]

not quite sure why the server linux CI fails (same on master branch), but I ran the test locally on my mac and it passes 100%

[aldehir]

ERROR unit/test_basic.py::test_server_start_simple - RuntimeError: Server process died with return code -4

Looks like the server process is getting killed with SIGILL (Illegal operation), -N maps to signal N according to https://docs.python.org/3/library/subprocess.html#subprocess.Popen.returncode

       Signal        x86/ARM     Alpha/   MIPS   PARISC   Notes
                   most others   SPARC
       ─────────────────────────────────────────────────────────────────
       SIGHUP           1           1       1       1
       SIGINT           2           2       2       2
       SIGQUIT          3           3       3       3
       SIGILL           4           4       4       4

[CISC]

Looks like the server process is getting killed with SIGILL (Illegal operation), -N maps to signal N according to https://docs.python.org/3/library/subprocess.html#subprocess.Popen.returncode

Yep, this is a ccache issue, I've deleted the caches and all is fine now.