[VTA] Support network which have no unique operator as start/stop name for graph...

[VTA] Support network which have no unique operator as start/stop name for graph pack. (#4703)

* [VTA] Support network which have no unique operator as start/stop name
for graph pack.

[Issue]
  Current vta use 'start' and 'stop' name to define the pack start point
  and end point, but this method not work for these network which have
  no 2 unique operator as  start point and stop point.

[Solution]
  In this solution we give 2 addtional parameters start_name_indx and
  stop_name_indx to make vta pack logic work with the said network,
  for exampl for following networks which have no unique operator,

  %0 = nn.add
  %1 = nn.conv2d
  %2 = nn.batch_norm
  %3 = nn.leaky_relu
  %4 = nn.add
  %5 = nn.conv2d
  %6 = nn.batch_norm
  %7 = nn.leaky_relu
  %8 = nn.add

  with this solution we can use following parameter format to make
  vta work on it.

  relay_prog = graph_pack(
                //....
                start_name="nn.add",
                stop_name="nn.add",
                start_name_idx=0,
                stop_name_idx=4)

  to apply on new network, by printing the network we can get index information like following.

  print(mod.astext(show_meta_data=False))
  relay_prog = graph_pack(mod
                          ...
                          start_name="nn.add",
                          stop_name="nn.add",
                          start_name_idx=0,
                          stop_name_idx=4)

* address review comments and fix index count bug

issue:
when do print(mod), the output not only the Call is also have other type
like Var, need add logic to count all except meta.

solution:
add related logic

* address review comments.

* address review comments

* add more detail comments.