[Transform] add ReplaceVerilogRelPaths transformation

this replaces relative paths with absolute ones for readmemh,
which causes trouble with pyverilator

src/finn/transformation/fpgadataflow/replace_verilog_relpaths.py

+import os
+
+import finn.custom_op.registry as registry
+import finn.util.basic as util
+from finn.transformation import Transformation
+
+
+class ReplaceVerilogRelPaths(Transformation):
+    """Convert ./ relative file paths to absolute ones for generated Verilog"""
+
+    def __init__(self):
+        super().__init__()
+
+    def apply(self, model):
+        for node in model.graph.node:
+            op_type = node.op_type
+            if node.domain == "finn":
+                backend_attribute = util.get_by_name(node.attribute, "backend")
+                if backend_attribute is None:
+                    continue
+                backend_value = backend_attribute.s.decode("UTF-8")
+                if backend_value == "fpgadataflow":
+                    try:
+                        # lookup op_type in registry of CustomOps
+                        inst = registry.custom_op[op_type](node)
+                        # find the IP gen dir
+                        ipgen_path = inst.get_nodeattr("ipgen_path")
+                        if ipgen_path is not None and os.path.isdir(ipgen_path):
+                            for dname, dirs, files in os.walk(ipgen_path):
+                                for fname in files:
+                                    if fname.endswith(".v"):
+                                        fpath = os.path.join(dname, fname)
+                                        with open(fpath, "r") as f:
+                                            s = f.read()
+                                        old = '$readmemh(".'
+                                        new = '$readmemh("%s' % dname
+                                        s = s.replace(old, new)
+                                        with open(fpath, "w") as f:
+                                            f.write(s)
+                    except KeyError:
+                        pass
+        return (model, False)