Add trivial inliner interfaces.

With this + manually setting private visibility on everything, a simple
classifier can be reduced to this IR, which is looking pretty lean and
mean:
https://gist.github.com/silvasean/19e7e2e21a61ff197aeac0dd864d188f

Also, include a utility script for importing `.pt` models.

```
pt_util.py --import classifier.pt | npcomp-opt -torch-globalize-object-graph
```

frontends/pytorch/utils/pt_util.py

@@ -0,0 +1,42 @@
+#!/usr/bin/env python3
+"""
+Utility for handling common tasks for exported `.pt` model files.
+
+Usage:
+    # Dump PyTorch data structures for .pt file.
+    # This does not involve any MLIR code.
+    $ pt_util.py --dump model.pt
+
+    # Import the .pt file into MLIR.
+    $ pt_util.py --import model.pt
+"""
+
+import torch
+import torch_mlir
+
+import argparse
+
+
+def main():
+    parser = argparse.ArgumentParser(
+        description="Utility for .pt files")
+    parser.add_argument("pt_file", metavar="PT_FILE", type=str,
+                        help="the .pt file to import")
+    parser.add_argument("--dump", action="store_true",
+                        help="dump the pytorch module")
+    parser.add_argument("--import", action="store_true",
+                        help="import the pytorch module")
+    args = parser.parse_args()
+    # TODO: Investigate why "cpu" is needed.
+    module = torch.jit.load(args.pt_file, map_location="cpu")
+    mb = torch_mlir.ModuleBuilder()
+    if args.dump:
+        module._c.dump(code=True, attrs=False, params=False)
+    # `import` is a Python keyword, so getattr is needed.
+    if getattr(args, "import", False):
+        mb.import_module(module._c)
+        mb.module.operation.print(large_elements_limit=16)
+
+
+if __name__ == "__main__":
+    main()

lib/Dialect/Basicpy/IR/BasicpyDialect.cpp

@@ -8,6 +8,7 @@
 
 #include "npcomp/Dialect/Basicpy/IR/BasicpyDialect.h"
 #include "mlir/IR/DialectImplementation.h"
+#include "mlir/Transforms/InliningUtils.h"
 #include "npcomp/Dialect/Basicpy/IR/BasicpyOps.h"
 #include "llvm/ADT/TypeSwitch.h"
 
@@ -15,6 +16,28 @@ using namespace mlir;
 using namespace mlir::NPCOMP;
 using namespace mlir::NPCOMP::Basicpy;
 
+//===----------------------------------------------------------------------===//
+// Dialect Interfaces
+//===----------------------------------------------------------------------===//
+
+namespace {
+struct BasicpyInlinerInterface : public DialectInlinerInterface {
+  using DialectInlinerInterface::DialectInlinerInterface;
+  bool isLegalToInline(Region *dest, Region *src, bool wouldBeCloned,
+                       BlockAndValueMapping &valueMapping) const final {
+    return true;
+  }
+  bool isLegalToInline(Operation *, Region *, bool wouldBeCloned,
+                       BlockAndValueMapping &) const final {
+    return true;
+  }
+};
+} // end anonymous namespace
+
+//===----------------------------------------------------------------------===//
+// Dialect Class
+//===----------------------------------------------------------------------===//
+
 void BasicpyDialect::initialize() {
   addOperations<
 #define GET_OP_LIST
@@ -22,6 +45,7 @@ void BasicpyDialect::initialize() {
       >();
   addTypes<BoolType, BytesType, DictType, EllipsisType, ListType, NoneType,
            SlotObjectType, StrType, TupleType, UnknownType>();
+  addInterfaces<BasicpyInlinerInterface>();
 
   // TODO: Make real ops for everything we need.
   allowUnknownOperations();

lib/Dialect/Torch/IR/TorchDialect.cpp

@@ -8,6 +8,7 @@
 
 #include "npcomp/Dialect/Torch/IR/TorchDialect.h"
 #include "mlir/IR/DialectImplementation.h"
+#include "mlir/Transforms/InliningUtils.h"
 #include "npcomp/Dialect/Torch/IR/TorchOps.h"
 #include "npcomp/Dialect/Torch/IR/TorchTypes.h"
 #include "llvm/ADT/StringExtras.h"
@@ -16,6 +17,24 @@
 using namespace mlir;
 using namespace mlir::NPCOMP::Torch;
 
+//===----------------------------------------------------------------------===//
+// Dialect Interfaces
+//===----------------------------------------------------------------------===//
+
+namespace {
+struct TorchInlinerInterface : public DialectInlinerInterface {
+  using DialectInlinerInterface::DialectInlinerInterface;
+  bool isLegalToInline(Region *dest, Region *src, bool wouldBeCloned,
+                       BlockAndValueMapping &valueMapping) const final {
+    return true;
+  }
+  bool isLegalToInline(Operation *, Region *, bool wouldBeCloned,
+                       BlockAndValueMapping &) const final {
+    return true;
+  }
+};
+} // end anonymous namespace
+
 //===----------------------------------------------------------------------===//
 // Tablegen Type Definitions
 //===----------------------------------------------------------------------===//
@@ -32,6 +51,7 @@ void TorchDialect::initialize() {
 #define GET_TYPEDEF_LIST
 #include "npcomp/Dialect/Torch/IR/TorchTypes.cpp.inc"
       >();
+  addInterfaces<TorchInlinerInterface>();
 }
 
 Type TorchDialect::parseType(DialectAsmParser &parser) const {