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fix: download Megatron C++ source files for compile_helpers()

The megatron-core pip package excludes the Makefile and helpers.cpp
needed by compile_helpers(). Download these two files from GitHub
at runtime before compilation. Also add pybind11 as an explicit
dependency since it's needed by the Makefile but only included as
a build-time dependency in the pip package.

Signed-off-by: XploY04 <2004agarwalyash@gmail.com>