When applying `ReadInstruction` to custom load it's not DatasetDict but list of Dataset?
Hi! You can get a `DatasetDict` if you pass a dictionary with read instructions as follows:
```python
instructions = [
    ReadInstruction(split_name="train", from_=0, to=10, unit='%', rounding='closest'),
    ReadInstruction(split_name="dev", from_=0, to=10, unit='%', rounding='closest'),
    ReadInstruction(split_name="test", from_=0, to=5, unit='%', rounding='closest')
]

dataset = load_dataset('csv', data_dir="data/", data_files={"train":"train.tsv", "dev":"dev.tsv", "test":"test.tsv"}, delimiter="\t", split={inst.split_name: inst for inst in instructions})
```

### Describe the bug

I am loading datasets from custom `tsv` files stored locally and applying split instructions for each split. Although the ReadInstruction is being applied correctly and I was expecting it to be `DatasetDict` but instead it is a list of `Dataset`. 

### Steps to reproduce the bug

Steps to reproduce the behaviour:

1. Import 
   `from datasets import load_dataset, ReadInstruction`
2. Instruction to load the dataset
  ```
instructions = [
      ReadInstruction(split_name="train", from_=0, to=10, unit='%', rounding='closest'),
      ReadInstruction(split_name="dev", from_=0, to=10, unit='%', rounding='closest'),
      ReadInstruction(split_name="test", from_=0, to=5, unit='%', rounding='closest')
  ]
```
3. Load 
`dataset = load_dataset('csv', data_dir="data/", data_files={"train":"train.tsv", "dev":"dev.tsv", "test":"test.tsv"}, delimiter="\t", split=instructions)`


### Expected behavior

**Current behaviour**

![Screenshot from 2023-01-16 10-45-27](https://user-images.githubusercontent.com/25720695/212614754-306898d8-8c27-4475-9bb8-0321bd939561.png)
:
 
**Expected behaviour**

![Screenshot from 2023-01-16 10-45-42](https://user-images.githubusercontent.com/25720695/212614813-0d336bf7-5266-482e-bb96-ef51f64de204.png)


### Environment info

``datasets==2.8.0
``
`Python==3.8.5
`
`Platform - Ubuntu 20.04.4 LTS`