input: Write a python function to create a bar plot using Plotly Express with a given DataFrame containing 'year' and 'salary' columns.
  assert create_bar_plot(pd.DataFrame({'year': [2020, 2021], 'salary': [50000, 55000]})) is not None
  assert create_bar_plot(pd.DataFrame({'year': [2018, 2019, 2020], 'salary': [40000, 45000, 50000]})) is not None
  assert create_bar_plot(pd.DataFrame({'year': [2015, 2016, 2017, 2018], 'salary': [30000, 32000, 34000, 36000]})) is not None
output: def create_bar_plot(df):
      import plotly.express as px
      fig = px.bar(df, x='year', y='salary')
      return fig