[jit] Add default values in script

[driazati]

PR adds support for int and float default values for script functions (values get cast to Tensor implicitly). Resolves expressions via eval (but only for types that can convert to a Tensor)

@apaszke @zdevito @ezyang

[zdevito]

Adding support for default arguments is a great thing to work on, but I think this is ultimately not the right approach. Instead, I would suggest a much simpler set of changes:

1. Extract the default values from the original functions using: https://stackoverflow.com/questions/12627118/get-a-function-arguments-default-value
2. Compile the function into a graph as is, without changing the compiler or ir.h, deriving a script::Method object without any default values in it. Use the types inside of the derived FunctionSchemaobject and inline IValue toIValue(py::handle obj, const TypePtr& type) to turn the python default value into an IValue.
3. Generate a new FunctionSchema object that includes the default IValues, and call setSchema on the Method to install it.

This approach is more modular (it doesn't require changes to AST parsing, the compiler, or the IR), and it relies on components we already have to robustly convert python values into IValue objects.