`accum_param_gradients!` does not support `scale_factor` for static functions

[ztangent]

As stated in the title, accum_param_gradients! does not support scale_factor for static functions. Calling accum_param_gradients! with a third argument returns ERROR: Not implemented, because it defaults to the abstract GFI definition.

This is due to (1) the lack of a generated method definition with the appropriate signature:

Gen.jl/src/static_ir/backprop.jl

Lines 508 to 512 in e5ed96f

	push!(generated_functions, quote
	@generated function $(GlobalRef(Gen, :accumulate_param_gradients!))(trace::T, retval_grad) where {T<:$(QuoteNode(StaticIRTrace))}
	$(QuoteNode(codegen_accumulate_param_gradients!))(trace, retval_grad)
	end
	end)

And (2) the lack of logic to handle a scale factor in the backward pass for trainable parameter nodes:

Gen.jl/src/static_ir/backprop.jl

Lines 169 to 185 in e5ed96f

	function back_codegen!(stmts, ir, selected_calls, fwd_marked, back_marked, node::TrainableParameterNode, mode)
	# handle case when it is the return node
	if node === ir.return_node && node in fwd_marked
	@assert node in back_marked
	push!(stmts, :(isnothing(retval_grad) && error("Required return value gradient but got nothing")))
	push!(stmts, :($(gradient_var(node)) += retval_grad))
	end
	if node in fwd_marked && node in back_marked
	cur_param_grad = :($(QuoteNode(get_param_grad))(trace.$static_ir_gen_fn_ref,
	$(QuoteNode(node.name))))
	push!(stmts, :($(QuoteNode(set_param_grad!))(trace.$static_ir_gen_fn_ref,
	$(QuoteNode(node.name)),
	$cur_param_grad + $(gradient_var(node)))))
	end
	end

[marcoct]

This was addressed by #417. However, I don't see a test for it here: https://github.com/probcomp/Gen.jl/blob/20210512-marcoct-gradopts/test/static_ir/gradients.jl.