metal : add POOL2D and fix IM2COL (#9943)

* add pool_2d

Signed-off-by: Junhee Yoo <junhee.yoo@navercorp.com>

* fix im2col and add unittest for N>=1024

Signed-off-by: Junhee Yoo <junhee.yoo@navercorp.com>

* add tests for N % 1024 != 0

Signed-off-by: Junhee Yoo <junhee.yoo@navercorp.com>

* remove trailing whitespaces

Signed-off-by: Junhee Yoo <junhee.yoo@navercorp.com>

* apply suggestions

Signed-off-by: Junhee Yoo <junhee.yoo@navercorp.com>

* apply more optimization

- original IM2COL kernel + _ext with MIN()

Signed-off-by: Junhee Yoo <junhee.yoo@navercorp.com>

* apply review: change kernel name of pool_2d

Signed-off-by: Junhee Yoo <junhee.yoo@navercorp.com>

* apply review

Signed-off-by: Junhee Yoo <junhee.yoo@navercorp.com>

* fix more formatting and enhance readability

Signed-off-by: Junhee Yoo <junhee.yoo@navercorp.com>

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Signed-off-by: Junhee Yoo <junhee.yoo@navercorp.com>