Thanks a lot!
It is defined in the open source code of RefineNet implemented by DrSleep:
class ResizeShorterScale(object):
“”“Resize shorter side to a given value and randomly scale.”""
def __init__(self, shorter_side, low_scale, high_scale):
    assert isinstance(shorter_side, int)
    self.shorter_side = shorter_side
    self.low_scale = low_scale
    self.high_scale = high_scale

def __call__(self, sample):
    image, mask = sample["image"], sample["mask"]
    min_side = min(image.shape[:2])
    scale = np.random.uniform(self.low_scale, self.high_scale)
    if min_side * scale < self.shorter_side:
        scale = self.shorter_side * 1.0 / min_side
    image = cv2.resize(
        image, None, fx=scale, fy=scale, interpolation=cv2.INTER_CUBIC
    )
    mask = cv2.resize(
        mask, None, fx=scale, fy=scale, interpolation=cv2.INTER_NEAREST
    )
    return {"image": image, "mask": mask}