Use GetValidName() throughout Python bindings

[rcurtin]

@NippunSharma added a function called GetValidName(), which maps binding parameters from their given name to a name suitable for Python:

mlpack/src/mlpack/bindings/python/wrapper_functions_impl.hpp

Lines 34 to 43 in 3f37143

	inline std::string GetValidName(const std::string& paramName)
	{
	std::string correctParamName = paramName;
	if(paramName == "lambda") correctParamName = "lambda_";
	else if(paramName == "input") correctParamName = "input_";
	else correctParamName = paramName;
	return correctParamName;
	}

Specifically, this maps lambda to lambda_, since lambda is a reserved keyword in Python, and it also maps input to input_, since input() is a builtin function in Python.

This PR changes all of our documentation functions in Python to use GetValidName(), instead of manually making the same change for lambda to lambda_ everywhere. After this PR is merged, the documentation will correctly reflect the input_ name instead of input; that is, the documentation is currently being printed incorrectly. It confused me for a few minutes while I was debugging the Python bindings. :)

[mlpack-bot]

Second approval provided automatically after 24 hours. 👍