[FEA] groupby-transform with `'diff'`
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feature request
Narwhals
Python
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Description
**Is your feature request related to a problem? Please describe.**
```python
df = cudf.DataFrame({'a': [1,1,2], 'b': [4,5,6]})
df.groupby('a')['b'].transform('diff')
```
**Additional context**
So we can support `nw.col('b').diff().over('a')` in Narwhals
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