Rename tests to match the code they test

- `module_import -> ivalue_import`, as it mainly tests ivalue_importer.cpp
- `graph_import -> node_import`, as it mainly tests node_importer.cpp
 - graph_importer.cpp does call into node_importer.cpp, but doesn't do
 much.

This was getting pretty confusing. Also add README.md's in each
directory for more clarity.

frontends/pytorch/test/ivalue_import/README.md

@@ -0,0 +1,10 @@
+# ivalue_import
+
+Most of the tests in this directory test importing of TorchScript
+`torch::jit::Module`'s.
+
+Modules are just one of many types of c10::IValue's and recursively contain
+c10::IValue's. Thus, the work of importing TorchScript modules is mainly
+about importing the wide variety of possible c10::IValue's, hence the name
+of this directory and the corresponding code in ivalue_importer.cpp that it
+exercises.

frontends/pytorch/test/node_import/README.md

@@ -0,0 +1,16 @@
+# node_import
+
+Most of the tests in this directory test the importing of TorchScript
+`torch::jit::Graph`'s.
+
+However, TorchScript graphs don't really correspond directly to anything on
+the MLIR side. They are a weird combination of a context, builder, and
+function and just holds a `torch::jit::Block`. It is `torch::jit::Node`
+and `torch::jit::Block` which form the recursive structure analogous to
+MLIR's operation/region/block.
+
+- `torch::jit::Node` == `mlir::Operation`,
+- `torch::jit::Block` == `mlir::Region` containing single `mlir::Block`
+
+Hence the name of this directory and the corresponding code in
+node_importer.h/cpp.