How to calculate date difference from two columns but with different rows and a condition?

Based on the example of dataframe below, I would like to calculate difference between two datetime for certain index and its cumulative. The expected results are as in the column diff_days and cum_diff days

index	date_a	date_b	diff_days	cum_diff_days
1	1/1/2023	NaT	NaT	-
1	NaT	NaT	NaT	-
1	NaT	3/1/2023	2	2
2	4/1/2023	NaT	NaT	-
2	NaT	NaT	NaT	-
2	NaT	6/1/2023	2	4
3	7/1/2023	NaT	NaT	-
3	NaT	8/1/2023	1	5
3	9/1/2023	NaT	NaT	-
3	NaT	NaT	NaT	-
3	NaT	11/1/2023	2	7

I have checked the other post where it calculates the difference between two dates, unfortunately that one is when the date is in the same row. For my case, I wanted to understand how to calculate the dates if it's on different rows at different column since just subtracting it with df['diff_days'] = df['date_a'] - df['date_b'] will produce aNaTresults. I really appreciate if there is someone enlighten me on this problem.