I have a `list` which is composed of `dict` of same structure ,

```
sample = [{'a':1, 'b':2, 'c':3}, {'a':1, 'b':2, 'c':4}, {'a':2, 'b':2, 'c':5}, {'a':2, 'b':3, 'c':5}]

```

I want to combine them by key `a`, the out should be

```
[{'a': 1, 'd': [{'b':2, 'c':3}, {'b':2, 'c':4}]}, {'a': 2, 'd': [{'b':2, 'c':5}, {'b': 3, 'c':5}]}]

```