* added mse_loss, nll_loss and kl_div_loss
* fixed axis not defined error in nll_loss
* fixed axis not defined in kl_div_loss
* added tests for mse, nll and kl_div
* modified docstrings and added reduce helper func
* updated docstring in kl_div_loss and moved helper func
* added new kl divergence implementation
* added reduction to test
* updated docstring of kl_div_loss with correct spelling
* added losses to nn.rst in docs
* Add notes about MacOS version restrictions for mlx in install docs
* Add notes about Xcode version requirements for building from source in install docs
* Let make detect the macosx sdk version being used
* Throw error if trying to build metal kernels with macOS <= 13.4
* Add metal-cpp for macOS 14.2
