Implements cosine decay with linear warmup

[YgorCastor]

This pull request introduces a new cosine decay schedule with linear warmup to the Polaris.Schedules module and adds corresponding tests to ensure its functionality.

New Feature: Cosine Decay Schedule with Linear Warmup

• lib/polaris/schedules.ex: Added the warmup_cosine_decay function, which implements a learning rate schedule that first increases linearly during a warmup period and then follows a cosine decay pattern. This function includes detailed documentation and options for customization.

Tests for New Feature

• test/polaris/schedules_test.exs: Added a series of tests for the warmup_cosine_decay function to verify its behavior, including tests for correct function arity, linear warmup phase, cosine decay phase, and respect for the alpha parameter.