deep-learning-saitama-way

docs: update README to reflect changes in directory structure, implementation status, and usage instructions for the `saitama_dl` package

test: add multi-element backward tests for addition and multiplication operations

fix: enhance zero_ and fill methods to ensure consistent return values and update backward method to initialize parent_grad correctly

fix: update fill method to accept a number for zeroing tensor data

feat: add simple linear regression example with forward and backward passes