input: Write a python function to add employee details to the dictionary.
  assert employee_details == {}
  add_employee('John Doe', 'john.doe@example.com', 'E001', 50000)
  assert employee_details == {'E001': {'name': 'John Doe', 'email': 'john.doe@example.com', 'salary': 50000}}
  add_employee('Jane Smith', 'jane.smith@example.com', 'E002', 60000)
  assert employee_details == {'E001': {'name': 'John Doe', 'email': 'john.doe@example.com', 'salary': 50000}, 'E002': {'name': 'Jane Smith', 'email': 'jane.smith@example.com', 'salary': 60000}}
output: employee_details = {}
  
  def add_employee(name, email, emp_id, salary):
      employee_details[emp_id] = {
          'name': name,
          'email': email,
          'salary': salary
      }