Support downloading specific splits in `load_dataset`
@CloseChoice This looks great! You're absolutely right about the missing comparison - that's a critical bug I missed. 
This is PR builds on top of #7706 to revive the unfinished #6832 but isn't just cleaning up things, here are some important changes:
 - `download_mode="FORCE_REDOWNLOAD"` is interpreted as always creating a clean slate, that means that even if we already did:
 ```python
load_dataset("<name>")
load_dataset("<name>", split="train", download_mode="force_redownload")
```
This makes sure that only the train dataset is available after executing both. This was different in the original PR, which proposed that train and test would be available. 
- `download_mode="REUSE_DATASET_IF_EXISTS"` is interpreted as only ever adding new data, never redownloading OR deleting other splits. This was different in the original PR, where  
 ```python
load_dataset("<name>", split="test")
load_dataset("<name>", split="train")
```
resulted in only the train data being available, which I deem very unintuitive and probably not what users want. Also I argue that this is just the first step to a more user friendly partial loading when specifying percentages (or maybe even single instances) via the ReadInstructions, and then doing
 ```python
load_dataset("<name>", split="test[:10%]")
load_dataset("<name>", split="test[10%:]")
```
should result IMO in the whole dataset being cached locally without redownloads.

Furthermore this PR fixes a couple issues with the previous PR, e.g. a [missing comparison](https://github.com/huggingface/datasets/pull/7706/files#diff-f933ce41f71c6c0d1ce658e27de62cbe0b45d777e9e68056dd012ac3eb9324f7R877) and adding tests for the proposed changes in behaviour, which would both fail on @ArjunJagdale's original PR. 

Todo:
 - [ ] update docs?

Future outlook (just my opinions and up for debate):
As mentioned before, I would see this as just a step towards the feature of partial percentage loading (though how the API should behave in that case is not entirely clear for me now) and maybe we could introduce another `download_mode="FORCE_REDOWNLOAD_SPLIT"`, which makes sure that even if a split is specified, only the referenced split is redownloaded and everything else left unchanged, this would then allow users more granular control over what they want to redownload. 
@lhoestq very curious to get your opinion on this.