[FEA] Rolling slope calculation with groupby

Hello.

I would like to calculate the rolling slope of y_value over x_value using cuML LinearRegression.

Sample data (cuDF dataframe):
```
| date | x_value | y_value |
| ------ | ------ | ---- |
| 2020-01-01 | 900 | 10 |
| 2020-01-01 | 905 | 15 |
| 2020-01-01 | 910 | 15 |
| 2020-01-01 | 915 | 15 |
| 2020-01-02 | 900 | 30 |
| 2020-01-02 | 905 | 40 |
| 2020-01-02 | 910 | 50 |
| ------ | ------ | ------ |
```
A simple function to use LinearRegression:
```
def RollingOLS(x, y):
lr = LinearRegression(fit_intercept = True, normalize = False, algorithm = 'svd')
reg = lr.fit(x, y)

return reg.coef_
```

What I would like to do:
```
data.groupby('date').rolling(2).apply(RollingOLS, x=x_value, y=y_value)
```

However, I am getting an error: ```NotImplementedError: Handling UDF with null values is not yet supported```. Is there any way to overcome this error? Thank you.