loader.cpp: support dumping IR/Graph visualizations from the commandline

[aweis]

This commit adds the following flags:

-dumpGraph - Prints Graph to stdout
-dumpGraphDAG=<file.dot> - Specify the file to export the Graph in DOT format
-dumpIR - Prints IR to stdout
-dumpIRDAG=<file.dot> - Specify the file to export the IR in DOT format

I had to slightly change the Module::dumpDAG() function to take in an
optional filepath. This is consistent with how the dumpDAG() function works
in Graph.cc.

[nadavrot]

These changes are looking good! I like that you've made the interface for the graph and the IR the same. This is good. I have a small request. Can you please add an overload to the function that accepts no parameter instead of a default argument? The reason is that when you call the function in the debugger (p G_->dumpDAG()) it gives and error, because LLDB does not fill in default arguments. Overloading the function would allow us to call the function from the debugger.

[aweis]

@nadavrot ^ I added the commit above with your suggestion. LMK if this is what you wanted and I will merge.

[nadavrot]

Nice!

[opti-mix]

Looks nice!

My only concern is: these flags can be useful not only inside the loader, but as general purpose flags to be used with Glow-based tools, e.g. if you debug tests, etc. Thus having the definitions of these command-line options inside loader.cpp is may be not such a great idea. May be they should be in IR.cpp and Graph.cpp respectively? IMHO, the loader should just use these flags, but not define them.

[nadavrot]

@opti-mix Roman, how will we know when to dump the DAG? In the loader we have a clear answer. In general code you never know if you want to dump before the auto-grad or after. Before optimizations or after. Before the target-specific-lowering or after, etc. I think that this is a good start.