sam : support box prompt (#1206)

sam : support box prompt (#1206)

* SAM: parse box prompt

* support box prompt

* sam: rename main.cpp to sam.cpp

* pass sam_prompt as const &

* support single mask output (default multimask)

* Update examples/sam/sam.cpp

Co-authored-by: Georgi Gerganov <redacted>
* Update examples/sam/sam.cpp

Co-authored-by: Georgi Gerganov <redacted>
* Update examples/sam/sam.cpp

Co-authored-by: Georgi Gerganov <redacted>
* Update examples/sam/sam.cpp

Co-authored-by: Georgi Gerganov <redacted>
* Update examples/sam/sam.cpp

Co-authored-by: Georgi Gerganov <redacted>
* removed debug print

* test sam example: box prompt and single mask output

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Co-authored-by: Georgi Gerganov <redacted>