ajax is returning 0

You need to follow these steps to properly call ajax function in WordPress,

1. Register and localize your script, eg.

   wp_register_script(
         '*handle_name*'
       , *js_file_path*
       , array(
           *dependency_name*
       )
       , true
       , true
   );
   

   It will load your script at page footer. Refer to the following link: https://developer.wordpress.org/reference/functions/wp_register_script/

   wp_localize_script(
         '*handle_name*'
       , '*a_unique_name_to_call_this_object*'
       , array(
           'ajaxurl' => admin_url( 'admin-ajax.php' ),
           'if_require_any_more_field' => 'necessary_value',
       )
   );
   

   It will create a js object with two properties, ajaxurl and if_require_any_more_field. Refer the following link: https://codex.wordpress.org/Function_Reference/wp_localize_script

2. Then Enqueue the script wp_enqueue_script('*handle_name*');

3. In your javascript/jQuery code

       var formDate = jQuery('#tpform1').serialize();                      
       var data = {
           action : 'trade_partners',
           form : formData
       }
   
       $.post(unique_object_name.ajaxurl, data, function(response) {
           alert(response);           
       });
   

4. Now time to write your php ajax calling function in functions.php file (for best practice)

   add_action('wp_ajax_trade_partners', 'custom_action');
   add_action('wp_ajax_nopriv_trade_partners', 'custom_action');     
   
   function custom_action() {
       global $wpdb;
       echo "<pre>";
       print_r($wpdb);
   }
   

If you use you write your js code within that php file, then you directly jumped into 3rd step, and within the script tag you need to create a variable which have the ajaxurl, ex: var ajaxurl = "<?php echo admin_url( 'admin-ajax.php' ); ?>"; and modify the line where actual ajax function is calling,

 $.post(ajaxurl, data, function(response) {
     alert(response);           
 });