CreateGridDimension.cs

public static Dimension CreateGridDimension(View view, Grid grid1, Grid grid2)
        {
            Curve curve1 = grid1.Curve;
            Curve curve2 = grid2.Curve;
            if(curve1 == null || curve1.Reference == null || curve2 == null || curve2.Reference == null)
            {
                return null;
            }
            Line line = Line.CreateBound(curve1.GetEndPoint(0), curve2.GetEndPoint(0));

            Reference re1 = new Reference(grid1);
            Reference re2 = new Reference(grid2);
            ReferenceArray references = new ReferenceArray();
            references.Append(re1);
            references.Append(re2);
            return view.Document.Create.NewDimension(view, line, references);
        }

https://chuongmep.com/Reference-Dim-Family-Dynamo

with python

import clr
clr.AddReference('RevitAPI')
from Autodesk.Revit.DB import *

clr.AddReference('RevitNodes')
import Revit
clr.ImportExtensions(Revit.GeometryConversion)
clr.ImportExtensions(Revit.Elements)

clr.AddReference('RevitServices')
import RevitServices
from RevitServices.Persistence import DocumentManager
from RevitServices.Transactions import TransactionManager

doc = DocumentManager.Instance.CurrentDBDocument

#Converting input from Dynamo to Revit
line = IN[0].ToRevitType()
grids = UnwrapElement(IN[1])

#Getting refrences from grid
gridRef = ReferenceArray()
opt = Options()
opt.ComputeReferences = True
opt.IncludeNonVisibleObjects = True
opt.View = doc.ActiveView
for grid in grids:
	for obj in grid.get_Geometry(opt):
		if isinstance(obj, Line):
			gline = obj
			gridRef.Append(gline.Reference)

#Create the dimension in a transaction
TransactionManager.Instance.EnsureInTransaction(doc)

dim = doc.Create.NewDimension(doc.ActiveView, line, gridRef)

TransactionManager.Instance.TransactionTaskDone()

OUT = dim