Support automatically Name Loop Variable in IRBuilder (#716) (#741)

* [SCHEDULE]enable partition const loop with build flag (#719)

    * enable partition loop with build flag

    * add a testcase, and modify LoopPartition related cases

*     * add document for split_const_loop

* [IRbuild]Support automatically Name Loop Variable in IRBuilder (#719)

    * add idx_num in class

* using typical index [i, j, k] first, then i_suffix

* keep inputs names

* fix lint

* improve comment of name

* fix lint

python/tvm/ir_builder.py

@@ -97,6 +97,7 @@ class IRBuilder(object):
     """
     def __init__(self):
         self._seq_stack = [[]]
+        self.nidx = 0
 
     def _pop_seq(self):
         """Pop sequence from stack"""
@@ -167,7 +168,8 @@ class IRBuilder(object):
             The end iteration scope
 
         name : str, optional
-            The name of iteration variable
+            The name of iteration variable, if no input names,
+            using typical index names i, j, k, then i_nidx
 
         dtype : str, optional
             The data type of iteration variable.
@@ -189,6 +191,9 @@ class IRBuilder(object):
             with ib.for_range(1, 10, name="i") as i:
                 x[i] = x[i - 1] + 1
         """
+        if name == 'i':
+            name = chr(ord(name) + self.nidx) if self.nidx < 3 else name + "_" + str(self.nidx - 3)
+            self.nidx += 1
         self._seq_stack.append([])
         loop_var = _api.var(name, dtype=dtype)
         extent = end if begin == 0 else _pass.Simplify(end - begin)