Fix the bug of overflow in AutoContrast

[SCZwangxiao]

Bug description

Fix the bug of data overflow in AutoContrast function when applying Randaugment. See this issue.

Commit explanation

The root cause is the overflow caused by np.uint8. Specifically, in L40 of the following code:

LAVIS/lavis/processors/randaugment.py

Lines 19 to 49 in ac8fc98

	def autocontrast_func(img, cutoff=0):
	"""
	same output as PIL.ImageOps.autocontrast
	"""
	n_bins = 256
	def tune_channel(ch):
	n = ch.size
	cut = cutoff * n // 100
	if cut == 0:
	high, low = ch.max(), ch.min()
	else:
	hist = cv2.calcHist([ch], [0], None, [n_bins], [0, n_bins])
	low = np.argwhere(np.cumsum(hist) > cut)
	low = 0 if low.shape[0] == 0 else low[0]
	high = np.argwhere(np.cumsum(hist[::-1]) > cut)
	high = n_bins - 1 if high.shape[0] == 0 else n_bins - 1 - high[0]
	if high <= low:
	table = np.arange(n_bins)
	else:
	scale = (n_bins - 1) / (high - low)
	offset = -low * scale
	table = np.arange(n_bins) * scale + offset
	table[table < 0] = 0
	table[table > n_bins - 1] = n_bins - 1
	table = table.clip(0, 255).astype(np.uint8)
	return table[ch]
	channels = [tune_channel(ch) for ch in cv2.split(img)]
	out = cv2.merge(channels)
	return out

where ch is one channel of an image (thus type np.uint8), and low is of type np.uint8. Therefore offset = -low * scale will cause an overflow since it cannot represent a negative number.

The reason why this bug did not affect much on the performance is: low is 0 most of the time.

[salesforce-cla]

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[SCZwangxiao]

salesforce-cla

Signed.