Add `__add__()`, `__iadd__()` and similar to `Dataset` class
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I was going through the codebase, and I believe the implementation of __add__() and __iadd__() will be similar to concatenate_datasets() after the elimination of code for arguments other than the list of datasets (info, split, axis). 
(Assuming elimination of axis means concatenating over axis 1.)
2021-12-24T10:59:49Z
https://github.com/huggingface/datasets/issues/3449#issuecomment-1000790041
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2021-12-24T11:00:07Z
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Most data frame libraries (Polars, Pandas, etc.) override `__add__` to perform (mathematical) summation, so having different behavior could lead to confusion.
2023-07-25T15:33:56Z
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2024-02-29T16:47:56Z
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**Is your feature request related to a problem? Please describe.**
No.

**Describe the solution you'd like**
I would like to be able to concatenate datasets as follows:
```python
>>> dataset["train"] += dataset["validation"]
```

... instead of using `concatenate_datasets()`:
```python
>>> raw_datasets["train"] = concatenate_datasets([raw_datasets["train"], raw_datasets["validation"]])
>>> del raw_datasets["validation"]
```

**Describe alternatives you've considered**
Well, I have considered `concatenate_datasets()` 😀

**Additional context**
N.a.
