input param not working?

arithmetic.py

import tensorflow as tf

a = tf.placeholder(tf.int32, name = 'a')
b = tf.placeholder(tf.int32, name = 'b')

add = tf.add(a, b, name = 'add')

tf.initialize_variables(tf.all_variables(), name = 'init')
definition = tf.Session().graph_def
tf.train.write_graph(definition, 'models', 'arithmetic.pb', as_text=False)

arithmetic.rs

extern crate tensorflow;

use tensorflow::{Session, SessionOptions, Step, Tensor, DataType};
use std::fs::File;
use std::io::Read;


fn main() {
    let filename = "examples/models/arithmetic.pb";

    let mut a = Tensor::<i32>::new(&[1]);
    a[0] = 3;
    let mut b = Tensor::<i32>::new(&[1]);
    b[0] = 5;

    // Initialize Session and load the model.
    let mut session = Session::new(&SessionOptions::new()).unwrap();
    let mut model = Vec::new();
    File::open(filename).unwrap().read_to_end(&mut model).unwrap();
    session.extend_graph(&model).unwrap();

    // Set A and B values
    let mut init_step = Step::new();
    init_step.add_input("a", &a).unwrap();
    init_step.add_input("b", &b).unwrap();
    init_step.add_target("init").unwrap();
    session.run(&mut init_step).unwrap();

    // Ask for the output and run
    let mut out_step = Step::new();
    let add = out_step.request_output("add").unwrap();
    session.run(&mut out_step).unwrap();

    // Print the result
    let result: i32 = out_step.take_output(add).unwrap().data()[0];
    println!("{:?}", result);
}

output

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: {inner:0x7f8b4a2011f0, InvalidArgument: You must feed a value for placeholder tensor 'a' with dtype int32
	 [[Node: a = Placeholder[dtype=DT_INT32, shape=[], _device="/job:localhost/replica:0/task:0/cpu:0"]()]]}', ../src/libcore/result.rs:788
stack backtrace:
   1:        0x1019fdb59 - std::sys::backtrace::tracing::imp::write::h00e948915d1e4c72
   2:        0x1019ffc40 - std::panicking::default_hook::_{{closure}}::h7b8a142818383fb8
   3:        0x1019ff0e0 - std::panicking::default_hook::h41cf296f654245d7
   4:        0x1019ff6a6 - std::panicking::rust_panic_with_hook::h4cbd7ca63ce1aee9
   5:        0x1019ff4f4 - std::panicking::begin_panic::h93672d0313d5e8e9
   6:        0x1019ff452 - std::panicking::begin_panic_fmt::hd0daa02942245d81
   7:        0x1019ff3b7 - rust_begin_unwind
   8:        0x101a258a0 - core::panicking::panic_fmt::hbfc935564d134c1b
   9:        0x1019eb4bc - core::result::unwrap_failed::hb73f4a5c3aa2e2c8
  10:        0x1019e8f0e - _<core..result..Result<T, E>>::unwrap::hbc6e44d6fbbca03a
  11:        0x1019ee10b - arithmetic::main::h312dd8515f8f6ba1
  12:        0x101a0022a - __rust_maybe_catch_panic
  13:        0x1019febae - std::rt::lang_start::h53bf99b0829cc03c
  14:        0x1019ee469 - main
error: Process didn't exit successfully: `target/debug/examples/arithmetic` (exit code: 101)