Minor fix to `MSELoss` docstring (#24077)
Minor fix to `MSELoss` docstring (#24077)

* Indent MSELoss example docs

* Point out input tensors should be of same shape

test=develop

* Document `MSELoss` input and return parameters

test=develop,test=document_fix
luotao1/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,luotao1/Paddle,luotao1/Paddle,PaddlePaddle/Paddle