Add spam dataloader

[Prateek0xeo]

This PR refactors the spam_dataloader function and introduces robust unit tests to ensure it meets expected functionality.

Results:

All test cases pass, confirming expected behavior of the dataloader under various scenarios.
Ensures compatibility with the Spam dataset structure and Torch requirements.

[dfalbel]

Thanks for the PR @Prateek0xeo !
I added some review comments.

[Prateek0xeo]

@dfalbel Thank you for the feedback!

1. As requested I have removed the library(torch) from the PR

2. Now I am allowing the user to define how they want to create the dataloader by keeping the dataset definition separate from the dataloader creation.
   By returning a torch::dataset object, the user has full control to decide:

• Batch size.
• Whether to shuffle the data.
• Number of parallel workers.

ds <- spam_dataset(download = TRUE)
trying URL 'https://hastie.su.domains/ElemStatLearn/datasets/spam.data'
Content type 'unknown' length 698341 bytes (681 KB)
downloaded 681 KB

loader <- torch::dataloader(
dataset = ds,
batch_size = 32,
shuffle = TRUE,
num_workers = 4
)
batch <- dataloader_make_iter(loader) %>% dataloader_next()
dim(batch$x)
[1] 32 57
length(batch$y)
[1] 32

If additional modifications are needed, please let me know!

[dfalbel]

Hi @Prateek0xeo

Thanks for updating the PR! I added a couple more comments.
Can you also add the dataset to the Readme table?

[dfalbel]

You don't need this require statement here. devtools::test() will make sure testthat is laoded.

[dfalbel]

You probably need to update the test cases, as spam_dataloader is now called spam_dataset.

[dfalbel]

This is likely to work in most scenarios because torch would already be loaded but in any case, we should prefix the call with:

Suggested change

	x = torch_tensor(x, dtype = torch_float()),
	y = torch_tensor(y, dtype = torch_long())
	x = torch::torch_tensor(x, dtype = torch_float()),
	y = torch::torch_tensor(y, dtype = torch_long())

[Prateek0xeo]

@dfalbel I have updated the PR in accordance to the new requests made