Fix: unexpected behaviours (bugs) in train_test_split fixed

[deependujha]

Before submitting

- [ ] Was this discussed/agreed via a Github issue? (no need for typos and docs improvements) - [ ] Did you read the [contributor guideline](https://github.com/Lightning-AI/lit-data/blob/main/.github/CONTRIBUTING.md), Pull Request section? - [ ] Did you make sure to update the docs? - [ ] Did you write any new necessary tests?

What does this PR do?

train_test_split works perfectly when asked to split dataset in splits=[0.1, 0.7, 0.2], but it fails when asked for splits=[0.1, 0.2, 0.7].

In the original code, this code will fail:

import os
from litdata import optimize, train_test_split, StreamingDataset, StreamingDataLoader

x, y, z = train_test_split(streaming_dataset=StreamingDataset("output_dir"), splits=[0.1, 0.2, 0.7])

print(f"{len(x)=}")
print(f"{len(y)=}")
print(f"{len(z)=}")

print(f"{x[:]=}")
print(f"{y[:]=}")
print(f"{z[:]=}") # this will raise error

x = StreamingDataLoader(x, batch_size=5)
y = StreamingDataLoader(y, batch_size=5)
z = StreamingDataLoader(z, batch_size=5)

print("-"*80)
print("iterate X")
for _x in x:
    print(_x)

print("-"*80)
print("iterate Y")
for _y in y:
    print(_y)

print("-"*80)
print("iterate Z")
for _z in z: # this will raise error
    print(_z)

print("-"*80)
print("All done!")

Except the failure, if you look at the values printed by y[:], it overlaps with x[:]. This was bcoz of the way reader was reading from chunks.

---

Code for output_dir

import os
from litdata import optimize, train_test_split, StreamingDataset

def compress(index):
    return (index, index ** 2)

optimize(
    fn=compress,
    inputs=list(range(100)),
    num_workers=4,
    output_dir="output_dir",
    chunk_bytes="64MB",
    mode="overwrite",
)

These bugs have been fixed in this PR. This PR originally aimed at closing a issue #186 , but it has been closed already, bcoz of some confusion.

PR review

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[tchaton]

Hey @deependujha. Can you describe which bugs this is fixing ?

[deependujha]

Hey @deependujha. Can you describe which bugs this is fixing ?

Sorry for the delay in response. I've updated the description. Plz have a look at it. It's an extension of a PR that has been merged already (#187 )