<td  class="slds-align-middle"><lightning:input /></td>

<td><span class="slds-align-middle"><lightning:input /></span></td>

<td Style="text-align: center;vertical-align:top;" valign="middle"><lightning:input /></td>

<table class="slds-table slds-table_bordered slds-table_cell-buffer slds-table_col-bordered">
     <thead>...</thead>
     <tbody>
      <tr> [td in the middle of the cell] </tr>
     </tbody>
    </table>