feat: Examples

[retraigo]

Examples:

• [x] Binary classification with Iris dataset.
• [x] Multiclass classification with iris dataset.
• [x] Binary text classification with SMS Spam Collection Dataset.
• [x] Multiple linear regression with student performance dataset.

Additional:

• [x] Expose the schedulers to mod.ts.
• [x] Fix log(0) in cross_entropy() and division by zero in cross_entropy_prime().

[retraigo]

There might still be something wrong with softmax / cross entropy. I'll make a commit later when I find it.