BUG: GroupBy.size created by TimeGrouper raises AttributeError
Related to #7453 2nd issue:
- `GroupBy.size` created by `TimeGrouper` raises `AttributeError`

ok can u add a bench for size in that case?
for both a regular groupby and TimeGrouper?
 OK, added benchmarks.
 can I post benchmarks for those 2? thxs
 Here it is. Cannot measure base performance of `groupby_dt_timegrouper_size` because of the bug, but it looks not slow comparing to `groupby_dt_size`

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
groupby_dt_size                              |  42.2956 |  55.4504 |   0.7628 |
groupby_dt_timegrouper_size                  |  34.2657 |  ------- |   ------ |
```
 run then with `-H`to test current perf (with your fix) with comparable samples (or do in ipython).
 Yep, current perf of `groupby_dt_timegrouper_size` is attached above.
